netlink: Undo const marker in netlink_is_kernel().

We can't do this without propagating the const to nlk_sk()
too, otherwise:

net/netlink/af_netlink.c: In function ‘netlink_is_kernel’:
net/netlink/af_netlink.c:103:2: warning: passing argument 1 of ‘nlk_sk’ discards ‘const’ qualifier from pointer target type [enabled by default]
net/netlink/af_netlink.c:96:36: note: expected ‘struct sock *’ but argument is of type ‘const struct sock *’

Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -98,7 +98,7 @@ static inline struct netlink_sock *nlk_sk(struct sock *sk)
 	return container_of(sk, struct netlink_sock, sk);
 }
 
-static inline int netlink_is_kernel(const struct sock *sk)
+static inline int netlink_is_kernel(struct sock *sk)
 {
 	return nlk_sk(sk)->flags & NETLINK_KERNEL_SOCKET;
 }