xfs: introduce new private btree cursor names

Just the defines of the new names - the conversion will be in
scripted commits after this.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: change "bc_bt" to "bc_ino"]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>