net: pass const to msg_data_left()

The msg_data_left() function doesn't modify the struct msghdr parameter,
so mark it as const. This allows the function to be used with const
references, improving type safety and making the API more flexible.

Signed-off-by: Breno Leitao <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

include/linux/socket.h

@@ -168,7 +168,7 @@ static inline struct cmsghdr * cmsg_nxthdr (struct msghdr *__msg, struct cmsghdr
 	return __cmsg_nxthdr(__msg->msg_control, __msg->msg_controllen, __cmsg);
 }
 
-static inline size_t msg_data_left(struct msghdr *msg)
+static inline size_t msg_data_left(const struct msghdr *msg)
 {
 	return iov_iter_count(&msg->msg_iter);
 }