xfs: move xfs_extent_free_defer_add to xfs_extfree_item.c

Move the code that adds the incore xfs_extent_free_item deferred work
data to a transaction to live with the EFI log item code.  This means
that the allocator code no longer has to know about the inner workings
of the EFI 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>