switch kernel_sendmsg() and kernel_recvmsg() to iov_iter_kvec()

For kernel_sendmsg() that eliminates the need to play with setfs();
for kernel_recvmsg() it does *not* - a couple of callers are using
it with non-NULL ->msg_control, which would be treated as userland
address on recvmsg side of things.

In all cases we are really setting a kvec-backed iov_iter, though.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

net/socket.c

@@ -627,18 +627,8 @@ EXPORT_SYMBOL(sock_sendmsg);
 int kernel_sendmsg(struct socket *sock, struct msghdr *msg,
 		   struct kvec *vec, size_t num, size_t size)
 {
-	mm_segment_t oldfs = get_fs();
-	int result;
-
-	set_fs(KERNEL_DS);
-	/*
-	 * the following is safe, since for compiler definitions of kvec and
-	 * iovec are identical, yielding the same in-core layout and alignment
-	 */
-	iov_iter_init(&msg->msg_iter, WRITE, (struct iovec *)vec, num, size);
-	result = sock_sendmsg(sock, msg, size);
-	set_fs(oldfs);
-	return result;
+	iov_iter_kvec(&msg->msg_iter, WRITE | ITER_KVEC, vec, num, size);
+	return sock_sendmsg(sock, msg, size);
 }
 EXPORT_SYMBOL(kernel_sendmsg);
 
@@ -755,12 +745,8 @@ int kernel_recvmsg(struct socket *sock, struct msghdr *msg,
 	mm_segment_t oldfs = get_fs();
 	int result;
 
+	iov_iter_kvec(&msg->msg_iter, READ | ITER_KVEC, vec, num, size);
 	set_fs(KERNEL_DS);
-	/*
-	 * the following is safe, since for compiler definitions of kvec and
-	 * iovec are identical, yielding the same in-core layout and alignment
-	 */
-	iov_iter_init(&msg->msg_iter, READ, (struct iovec *)vec, num, size);
 	result = sock_recvmsg(sock, msg, size, flags);
 	set_fs(oldfs);
 	return result;