net: make /proc/net/protocols namespace aware

Converting /proc/net/protocols to be namespace aware is quite easy
and permits us to use sock_prot_inuse_get().

This provides seperate counters for each protocol. For example
we can really count TCPv6 sockets and TCPv4 sockets, while previously,
we had the same value, and this value was not namespace aware.

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