net: inet_add_protocol() can use cmpxchg()

Use cmpxchg() to get rid of spinlocks in inet_add_protocol() and
friends.

inet_protos[] & inet6_protos[] are moved to read_mostly section

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/ipv4/protocol.c
Simple merge

diff --cc net/ipv6/protocol.c
Simple merge