devlink: call peernet2id_alloc() with net pointer under RCU read lock

peernet2id_alloc() allows to be called lockless with peer net pointer
obtained in RCU critical section and makes sure to return ns ID if net
namespaces is not being removed concurrently. Benefit from
read_pnet_rcu() helper addition, use it to obtain net pointer under RCU
read lock and pass it to peernet2id_alloc() to get ns ID.

Fixes: c137743b ("devlink: introduce object and nested devlink relationship infra")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>