xfs: attr leaf verifier needs to check for obviously bad count

xfs: attr leaf verifier needs to check for obviously bad count

Source kernel commit: 79a69bf8dc240ebeb105226a8a8540df136bf987

In the attribute leaf verifier, we can check for obviously bad values of
firstused and count so that later attempts at lasthash don't run off the
end of the memory buffer.  Found by ones fuzzing hdr.count in xfs/400 with
KASAN.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>