sock: make lockdep_sock_is_held static inline

I forgot to add inline to lockdep_sock_is_held, so it generated all
kinds of build warnings if not build with lockdep support.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1360,7 +1360,7 @@ do {									\
 	lockdep_init_map(&(sk)->sk_lock.dep_map, (name), (key), 0);	\
 } while (0)
 
-static bool lockdep_sock_is_held(const struct sock *csk)
+static inline bool lockdep_sock_is_held(const struct sock *csk)
 {
 	struct sock *sk = (struct sock *)csk;