btrfs-progs: fi-usage: Fix wrong RAID10 used and unallocated space

[BUG]
For a very basic RAID10 with 4 disks, "fi usage" and "fi show" are outputting conflicting result:
------

 # btrfs fi show /mnt/btrfs/
Label: none  uuid: 6d0229db-28d1-4696-ac20-e828cc45dc40
	Total devices 4 FS bytes used 1.12MiB
	devid    1 size 5.00GiB used 2.01GiB path /dev/mapper/data-disk1
	devid    2 size 5.00GiB used 2.01GiB path /dev/mapper/data-disk2
	devid    3 size 5.00GiB used 2.01GiB path /dev/mapper/data-disk3
	devid    4 size 5.00GiB used 2.01GiB path /dev/mapper/data-disk4

Here the unallocated space for disk4 should be a little less than 3G. (5.00 Gib - 2.01GiB)

 # btrfs fi usage  /mnt/btrfs/
Overall:
    Device size:		  20.00GiB
    Device allocated:		   8.03GiB
    Device unallocated:		  11.97GiB
    Device missing:		     0.00B
    Used:			   2.25MiB
    Free (estimated):		   7.98GiB	(min: 7.98GiB)
    Data ratio:			      2.00
    Metadata ratio:		      2.00
    Global reserve:		  16.00MiB	(used: 0.00B)

Data,RAID10: Size:2.00GiB, Used:1.00MiB
...
/dev/mapper/data-disk4 512.00MiB

Metadata,RAID10: Size:2.00GiB, Used:112.00KiB
...
/dev/mapper/data-disk4 512.00MiB

System,RAID10: Size:16.00MiB, Used:16.00KiB
...
/dev/mapper/data-disk4 4.00MiB

Unallocated:
...
/dev/mapper/data-disk4 4.00GiB

While fi usage shows we still have 4.00GiB unallocated space.
------

[CAUSE]
calc_chunk_size() is used to convert chunk size to device extent size, which is used to get the per-device data/meta/sys used space.

However, for RAID10 we just divide the chunk size with num_stripes, without taking sub stripes into account.

Resulting data/meta/sys usage in RAID10 halved.

[FIX]
Take the missing sub stripes into consideration.

---
v2:
Nothing

---
 cmds-fi-usage.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 0b0e47fee194..0af4bdbc9370 100644
@@ -664,7 +664,7 @@ static u64 calc_chunk_size(struct chunk_info *ci)
 	else if (ci->type & BTRFS_BLOCK_GROUP_RAID6)
 		return ci->size / (ci->num_stripes -2);
 	else if (ci->type & BTRFS_BLOCK_GROUP_RAID10)
-		return ci->size / ci->num_stripes;
+		return ci->size / (ci->num_stripes / 2);
 	return ci->size;
 }
 

2.15.1

---
v2:
Don't create unnecessary temporary dir. Use variable directly to save output
Use run_check_mount_test_dev() macro with TEST_DEV set manually.

---
 tests/misc-tests/028-fi-usage-cross-check/test.sh | 44 +++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100755 tests/misc-tests/028-fi-usage-cross-check/test.sh

new file mode 100755
index 000000000000..0343888b3413
@@ -0,0 +1,44 @@
+#!/bin/bash
+# check fi-usage is output correct unallocated space for RAID10
+
+source "$TOP/tests/common"
+
+check_prereq mkfs.btrfs
+check_prereq btrfs
+
+setup_root_helper
+
+setup_loopdevs 4
+prepare_loopdevs
+dev1=${loopdevs[1]}
+
+run_check $SUDO_HELPER "$TOP/mkfs.btrfs" -d raid10 -m raid10 -f ${loopdevs[@]}
+TEST_DEV=$dev1
+run_check_mount_test_dev
+
+output=$(run_check_stdout $SUDO_HELPER "$TOP/btrfs" filesystem show --raw "$TEST_MNT" |\
+	 grep "$dev1")
+if [ -z "$output" ]; then
+	_fail "failed to get correct fi show output"
+fi
+
+dev1_total=$(echo $output | awk '{print $4}')
+dev1_used=$(echo $output | awk '{print $6}')
+dev1_fi_show_unallocated=$(( $dev1_total - $dev1_used))
+
+output=$(run_check_stdout $SUDO_HELPER "$TOP/btrfs" filesystem usage --raw "$TEST_MNT" |\
+	 grep "$dev1" | tail -n1)
+if [ -z "$output" ]; then
+	_fail "failed to get correct fi usage output"
+fi
+
+dev1_fi_usage_unallocated=$(echo $output | awk '{print $2}')
+echo "fi usage unallocated for devid1 is $dev1_fi_usage_unallocated" >> "$RESULTS"
+echo "fi show unallocated for devid1 is $dev1_fi_show_unallocated" >> "$RESULTS"
+
+if [ $dev1_fi_show_unallocated -ne $dev1_fi_usage_unallocated ]; then
+	_fail "fi usage unallocated mismatch with fi show"
+fi
+
+run_check $SUDO_HELPER umount "$TEST_MNT"
+cleanup_loopdevs

2.15.1