inet_diag: allow concurrent operations

inet_diag_lock_handler() current implementation uses a mutex
to protect inet_diag_table[] array against concurrent changes.

This makes inet_diag dump serialized, thus less scalable
than legacy /proc files.

It is time to switch to full RCU protection.

As a bonus, if a target is statically linked instead of being
modular, inet_diag_lock_handler() & inet_diag_unlock_handler()
reduce to reads only.

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>