btrfs: reloc: move error handling of build_backref_tree() to backref.c

The error cleanup will be extracted as a new function,
btrfs_backref_error_cleanup(), and moved to backref.c and exported for
later usage.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>