btrfs: reloc: refactor indirect tree backref processing into its own function

The processing of indirect tree backref (TREE_BLOCK_REF) is the most
complex work.

We need to grab the fs root, do a tree search to locate all its parent
nodes, link all needed edges, and put all uncached edges to pending edge
list.

This is definitely worth a helper function.

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>