test-appliance: support f2fs-tools v1.9 and later

Pass the -f option to mkfs.f2fs when it appears to support it. This is required by f2fs-tools v1.9 and later in order to format the filesystem even when an existing filesystem is detected. But earlier versions did not accept this option.

---
 kvm-xfstests/test-appliance/files/root/fs/f2fs/config | 4 ++++
 1 file changed, 4 insertions(+)

index 45fb2dd..0efcb76 100644
@@ -20,6 +20,10 @@ function format_filesystem()
     local dev="$1"
     local opts="$2"
 
+    if grep -q 'force overwrite' /sbin/mkfs.f2fs; then
+	# f2fs-tools v1.9+
+	opts+=" -f"
+    fi
     /sbin/mkfs.f2fs -q $opts "$dev"
     return $?
 }

2.17.0.484.g0c8726318c-goog

Applied for now, but ugh. Grepping strings out of binaries is not something I really like to depend upon. Can we convince the f2fs folks to provide a "/sbin/mkfs.f2fs -V" which prints a version string, or some such?

- Ted

Yes, it's ugly. As something maybe a bit better, I've proposed

	mkfs.f2fs --help |& grep -q "[[:space:]]-f[[:space:]|]"

in v2 of the xfstests patch to common/config. So if that gets accepted into xfstests I'll change this to use the same method.

Thanks,

Eric