xfs/288: filter out extra mkfs warning

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>