btrfs: refactor the delayed item deletion entry point

btrfs: refactor the delayed item deletion entry point

The delayed item deletion entry point, btrfs_delete_delayed_items(), is a
bit convoluted for a few reasons:

1) It's really a loop disguised with labels and goto statements;

2) There's a 'delete_fail' label which isn't only for error cases, we can
   jump to that label even if no error happened, if we simply don't have
   more delayed items to delete;

3) Unnecessarily keeps track of the current and previous items for no
   good reason, as after getting the next item and releasing the current
   one, it just jumps to the 'again' label just to look again for the
   first delayed item;

4) When a delayed item is not in the tree (because it was already deleted
   before), it releases the item while holding a path locked, which is
   not necessary and adds more contention to the tree, specially taking
   into account that the path came from a deletion search, meaning we have
   write locks for nodes at levels 2, 1 and 0. And releasing the item is
   not computationally trivial (rb tree deletion, a kfree() and some
   trivial things).

So refactor it to use a while loop and add some comments to make it more
obvious why we can have delayed items without a matching item in the tree
as well as why not keep the delayed node locked all the time when running
all its deletion items. This is also a preparation for some upcoming work
involving delayed items.

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