xfs: fixup xfs_attr_get_ilocked

The comment mentioned the wrong lock.  Also add an ASSERT to assert
this locking precondition.

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