xfs: make the btree cursor union members named structure

xfs: make the btree cursor union members named structure

Source kernel commit: 68422d90dad4fe98f99d6e414aeec9a58d5185d5

we need to name the btree cursor private structures to be able
to pull them out of the deeply nested structure definition they are
in now.

Based on code extracted from a patchset by Darrick Wong.

Signed-off-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>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>