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

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: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit ae928781)
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: Iccb1de9343520ec98f6cc100e211ccd9f67d2f4d