UPSTREAM: wireguard: use synchronize_net rather than synchronize_rcu

Many of the synchronization points are sometimes called under the rtnl
lock, which means we should use synchronize_net rather than
synchronize_rcu. Under the hood, this expands to using the expedited
flavor of function in the event that rtnl is held, in order to not stall
other concurrent changes.

This fixes some very, very long delays when removing multiple peers at
once, which would cause some operations to take several minutes.

Bug: 254441685
Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 24b70eeeb4f46c09487f8155239ebfb1f875774a)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I59dc02502d1597c4d05100a874b8a34d61e2eaa5