xfs: move xfs_bmap_defer_add to xfs_bmap_item.c

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

As a consequence, we can hide the _get_group helper.

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