wireguard: socket: remove extra call to synchronize_net

synchronize_net() is a wrapper around synchronize_rcu(), so there's no
point in having synchronize_net and synchronize_rcu back to back,
despite the documentation comment suggesting maybe it's somewhat useful,
"Wait for packets currently being received to be done." This commit
removes the extra call.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>