xfs: make the btree ag cursor private union anonymous

This is much less widely used than the bc_private union was, so this
is done as a single patch. The named union xfs_btree_cur_private
goes away and is embedded into the struct xfs_btree_cur_ag as an
anonymous union, and the code is modified via this script:

$ sed -i 's/priv\.\([abt|refc]\)/\1/g' fs/xfs/*[ch] fs/xfs/*/*[ch]

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>