xfs: make the btree cursor union members named structure

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>