net: merge net->core.prot_inuse and net->core.sock_inuse

net->core.sock_inuse is a per cpu variable (int),
while net->core.prot_inuse is another per cpu variable
of 64 integers.

per cpu allocator tend to place them in very different places.

Grouping them together makes sense, since it makes
updates potentially faster, if hitting the same
cache line.

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