btrfs: assert delayed refs lock is held at add_delayed_ref_head()

The delayed refs lock must be held when calling add_delayed_ref_head(),
so assert that it's being held.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>