A __rcu annotation got lost during refactoring, which caused sparse to
become enraged.

Fixes: bf7b042d ("wireguard: allowedips: free empty intermediate nodes when removing single node")
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>