net: packet: make pkt_sk() inline

It's better make 'pkt_sk()' inline here, as non-inline function
shouldn't occur in headers. Besides, this function is simple
enough to be inline.

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Link: https://lore.kernel.org/r/20210127123302.29842-1-dong.menglong@zte.com.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/packet/internal.h

@@ -139,7 +139,7 @@ struct packet_sock {
 	atomic_t		tp_drops ____cacheline_aligned_in_smp;
 };
 
-static struct packet_sock *pkt_sk(struct sock *sk)
+static inline struct packet_sock *pkt_sk(struct sock *sk)
 {
 	return (struct packet_sock *)sk;
 }