inet: prepare inet_base_seq() to run without RTNL

In the following patch, inet_base_seq() will no longer be called
with RTNL held.

Add READ_ONCE()/WRITE_ONCE() annotations in dev_base_seq_inc()
and inet_base_seq().

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