net: Introduce read_pnet() and write_pnet() helpers

This patch introduces two helpers that deal with reading and writing
struct net pointers in various network structures.

Their implementation depends on CONFIG_NET_NS

For symmetry, both functions work with "struct net **pnet".

Their usage should reduce the number of #ifdef CONFIG_NET_NS,
without adding many helpers for each network structure
that hold a "struct net *pointer"

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>