btrfs: backref: rename and move backref_tree_panic()

Also change the parameter, since all callers can easily grab an fs_info,
there is no need for all the pointer chasing.

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