xfs: fix acl count validation in xfs_acl_from_disk()

Commit fa8b18ed didn't prevent the integer overflow and possible
memory corruption.  "count" can go negative and bypass the check.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>