sock_diag: add SK_MEMINFO_DROPS

Reporting sk_drops to user space was available for UDP
sockets using /proc interface.

Add this to sock_diag, so that we can have the same information
available to ss users, and we'll be able to add sk_drops
indications for TCP sockets as well.

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