btrfs: batch up release of reserved metadata for delayed items used for deletion

With Filipe's recent rework of the delayed inode code one aspect which
isn't batched is the release of the reserved metadata of delayed inode's
delete items. With this patch on top of Filipe's rework and running the
same test as provided in the description of a patch titled
"btrfs: improve batch deletion of delayed dir index items" I observe
the following change of the number of calls to btrfs_block_rsv_release:

Before this change:
- block_rsv_release:                      1004
- btrfs_delete_delayed_items_total_time: 14602
- delete_batches:                          505

After:
- block_rsv_release:                       510
- btrfs_delete_delayed_items_total_time: 13643
- delete_batches:                          507

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>