xfs: scrub inode mode properly

Since we've used up all the bits in i_mode, the existing mode check
doesn't actually do anything useful.  However, we've not used all the
bit values in the format portion of i_mode, so we /do/ need to test
that for bad values.

Fixes: 80e4e126 ("xfs: scrub inodes")
Fixes-coverity-id: 1423992
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>