sock_diag: allow concurrent operation in sock_diag_rcv_msg()

TCPDIAG_GETSOCK and DCCPDIAG_GETSOCK diag are serialized
on sock_diag_table_mutex.

This is to make sure inet_diag module is not unloaded
while diag was ongoing.

It is time to get rid of this mutex and use RCU protection,
allowing full parallelism.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>