sock_diag: Introduce the meminfo nla core (v2)

Add a routine that dumps memory-related values of a socket.
It's made as an array to make it possible to add more stuff
here later without breaking compatibility.

Since v1: The SK_MEMINFO_ constants are in userspace
visible part of sock_diag.h, the rest is under __KERNEL__.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>