sock_diag: remove sock_diag_mutex

sock_diag_rcv() is still serializing its operations using
a mutex, for no good reason.

This came with commit 0a9c7301 ("[INET_DIAG]: Fix oops
in netlink_rcv_skb"), but the root cause has been fixed
with commit cd40b7d3 ("[NET]: make netlink user -> kernel
interface synchronious")

Remove this mutex to let multiple threads run concurrently.

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>