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>

net/core/sock_diag.c

@@ -292,13 +292,9 @@ static int sock_diag_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 	}
 }
 
-static DEFINE_MUTEX(sock_diag_mutex);
-
 static void sock_diag_rcv(struct sk_buff *skb)
 {
-	mutex_lock(&sock_diag_mutex);
 	netlink_rcv_skb(skb, &sock_diag_rcv_msg);
-	mutex_unlock(&sock_diag_mutex);
 }
 
 static int sock_diag_bind(struct net *net, int group)