btrfs: backref: introduce the skeleton of btrfs_backref_iter

btrfs: backref: introduce the skeleton of btrfs_backref_iter

Due to the complex nature of btrfs extent tree, when we want to iterate
all backrefs of one extent, this involves quite a lot of work, like
searching the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
backrefs.

Normally this would result in a complex code, something like:

  btrfs_search_slot()
  /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
  while (1) {	/* Loop for extent tree items */
	while (ptr < end) { /* Loop for inlined items */
		/* Real work here */
	}
  next:
  	ret = btrfs_next_item()
	/* Ensure we're still at keyed item for specified bytenr */
  }

The idea of btrfs_backref_iter is to avoid such complex and hard to
read code structure, but something like the following:

  iter = btrfs_backref_iter_alloc();
  ret = btrfs_backref_iter_start(iter, bytenr);
  if (ret < 0)
	goto out;
  for (; ; ret = btrfs_backref_iter_next(iter)) {
	/* Real work here */
  }
  out:
  btrfs_backref_iter_free(iter);

This patch is just the skeleton + btrfs_backref_iter_start() code.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>