xfs: split the per-btree union in struct xfs_btree_cur

Split up the union that encodes btree-specific fields in struct
xfs_btree_cur.  Most fields in there are specific to the btree type
encoded in xfs_btree_ops.type, and we can use the obviously named union
for that.  But one field is specific to the bmapbt and two are shared by
the refcount and rtrefcountbt.  Move those to a separate union to make
the usage clear and not need a separate struct for the refcount-related
fields.

This will also make unnecessary some very awkward btree cursor
refc/rtrefc switching logic in the rtrefcount patchset.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>