net: treat possible_net_t net pointer as an RCU one and add read_pnet_rcu()

Make the net pointer stored in possible_net_t structure annotated as
an RCU pointer. Change the access helpers to treat it as such.
Introduce read_pnet_rcu() helper to allow caller to dereference
the net pointer under RCU read lock.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>