inet: constify inet_sdif() argument

inet_sdif() does not modify the skb.

This will permit propagating the const qualifier in
udp{4|6}_lib_lookup_skb() functions.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NAlexander Lobakin <alobakin@pm.me>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

include/net/ip.h

@@ -99,7 +99,7 @@ static inline void ipcm_init_sk(struct ipcm_cookie *ipcm,
 #define PKTINFO_SKB_CB(skb) ((struct in_pktinfo *)((skb)->cb))
 
 /* return enslaved device index if relevant */
-static inline int inet_sdif(struct sk_buff *skb)
+static inline int inet_sdif(const struct sk_buff *skb)
 {
 #if IS_ENABLED(CONFIG_NET_L3_MASTER_DEV)
 	if (skb && ipv4_l3mdev_skb(IPCB(skb)->flags))