btrfs-progs: image: Only enlarge result image if it's regular file

[BUG]
misc-test/021 will fail with the following error message:

====== RUN CHECK root_helper btrfs-progs/btrfs-image -r btrfs-progs/tests//test.img /dev/loop2
ERROR: failed to enlarge result image: Invalid argument
ERROR: failed to fix chunks and devices mapping, the fs may not be mountable: Invalid argument extent buffer leak: start 30638080 len 16384
extent buffer leak: start 30408704 len 16384
WARNING: dirty eb leak (aborted trans): start 30408704 len 16384
ERROR: restore failed: Invalid argument
failed: root_helper btrfs-progs/btrfs-image -r btrfs-progs/tests//test.img /dev/loop2 test failed for case 021-image-multi-devices

[CAUSE]
For misc-test/021, we're using loopback device, which doesn't support ftruncate.
That's why it returns EINVAL.

[FIX]
Only call ftruncate64() if we're operating on a regluar file.

Fixes a7a44164c84e ("btrfs-progs: image: Use correct device size when restoring")

Signed-off-by Qu Wenruo <[email protected]>

---
 image/main.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

image/main.c +11 -3

index cff5cc87662c..4fba8283cec7 100644
@@ -2101,6 +2101,7 @@ static int fixup_device_size(struct btrfs_trans_handle *trans,
 	struct extent_buffer *leaf;
 	struct btrfs_root *root = fs_info->chunk_root;
 	struct btrfs_key key;
+	struct stat buf;
 	u64 devid, cur_devid;
 	u64 dev_size; /* Get from last dev extents */
 	int ret;

@@ -2147,12 +2148,19 @@ static int fixup_device_size(struct btrfs_trans_handle *trans,
 
 	btrfs_set_stack_device_total_bytes(dev_item, dev_size);
 	btrfs_set_stack_device_bytes_used(dev_item, mdres->alloced_chunks);
-	/* Don't forget to enlarge the real file */
-	ret = ftruncate64(out_fd, dev_size);
+	ret = fstat(out_fd, &buf);
 	if (ret < 0) {
-		error("failed to enlarge result image: %m");
+		error("failed to stat result image: %m");
 		return -errno;
 	}
+	if (S_ISREG(buf.st_mode)) {
+		/* Don't forget to enlarge the real file */
+		ret = ftruncate64(out_fd, dev_size);
+		if (ret < 0) {
+			error("failed to enlarge result image: %m");
+			return -errno;
+		}
+	}
 
 	key.objectid = BTRFS_DEV_ITEMS_OBJECTID;
 	key.type = BTRFS_DEV_ITEM_KEY;

This patch can also be folded into the original commit a7a44164c84e ("btrfs-progs: image: Use correct device size when restoring") in devel branch.