xfs: only allocate the buffer size actually needed in __xfs_set_acl

No need to allocate the max size if we can just allocate the easily
known actual ACL size.

Suggested-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>