backref cache

find_reloc_root() searches reloc_control::reloc_root_tree to find the
reloc root.
This behavior is only useful for relocation backref cache.

For the incoming more generic purposed backref cache, we don't care
about who owns the reloc root, but only care if it's a reloc root.

So this patch makes the following modifications to make the reloc root
search more specific to relocation backref:
- Add backref_node::is_reloc_root
  This will be an extra indicator for generic purposed backref cache.
  User doesn't need to read root key from backref_node::root to
  determine if it's a reloc root.

- Add backref_cache::is_reloc
  This will allow backref cache code to do different behavior for
  generic purposed backref cache and relocation backref cache.

- Make find_reloc_root() to accept fs_info
  Just a personal taste.

Signed-off-by: Qu Wenruo <wqu@suse.com>