xfs/288: filter out extra mkfs warning

From xfsprogs v4.7, mkfs.xfs add respecification detection by
commit 9090e18. Then mkfs will fail and return if we run it
as below:

  mkfs.xfs -m crc=1,finobt=1 -m crc=0 ....

Then _scratch_mkfs_xfs can deal with this problem. But for old
xfsprogs ( < v4.7), it replace the first "crc=1" with the second
"crc=0". Then "crc=0,finobt=1" cause a warning, but keep running:

  "warning: finobt not supported without CRC support, disabled."

This extra warning breaks the golden image of xfs/288, so filter
it out in case.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/xfs/288

@@ -58,7 +58,7 @@ if [ -z "$XFS_MKFS_HAS_NO_META_SUPPORT" ]; then
 	mkfs_opts="-m crc=0"
 fi
 # get block size ($dbsize) from the mkfs output
-_scratch_mkfs_xfs $mkfs_opts | _filter_mkfs 2>$tmp.mkfs >/dev/null
+_scratch_mkfs_xfs $mkfs_opts 2>/dev/null | _filter_mkfs 2>$tmp.mkfs >/dev/null
 . $tmp.mkfs
 
 _scratch_mount