xfs: move xfs_refcount_update_defer_add to xfs_refcount_item.c

Move the code that adds the incore xfs_refcount_update_item deferred
work data to a transaction live with the CUI log item code.  This means
that the refcount code no longer has to know about the inner workings of
the CUI log items.

As a consequence, we can get rid of the _{get,put}_group helpers.

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