UPSTREAM: 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>
(cherry picked from commit 1fbc33b0a7feb6ca72bf7dc8a05d81485ee8ee2e)
Bug: 152722841
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I8f152ba49c926f249c4e0fd0f9bace30df269d5d