UPSTREAM: wireguard: allowedips: add missing __rcu annotation to satisfy sparse

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

Bug: 254441685
Fixes: bf7b042dc62a ("wireguard: allowedips: free empty intermediate nodes when removing single node")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit ae9287811ba75571cd69505d50ab0e612ace8572)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: Icf700db89dbba3ba4026b94722b291121b5ef01f