net: use synchronize_rcu_expedited in cleanup_net()

cleanup_net() is calling synchronize_rcu() right before
acquiring RTNL.

synchronize_rcu() is much slower than synchronize_rcu_expedited(),
and cleanup_net() is currently single threaded. In many workloads
we want cleanup_net() to be fast, in order to free memory and various
sysfs and procfs entries as fast as possible.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>