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 purpose 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.
  Also for reloc tree root, it's useless and will be queued to useless
  list.

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

- Pass fs_info to find_reloc_root()

- Export find_reloc_root()
  So backref.c can utilize this 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>