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

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>