net: kcm: fix memory leak in kcm_sendmsg

Syzbot reported memory leak in kcm_sendmsg()[1].
The problem was in non-freed frag_list in case of error.

In the while loop:

	if (head == skb)
		skb_shinfo(head)->frag_list = tskb;
	else
		skb->next = tskb;

frag_list filled with skbs, but nothing was freeing them.

backtrace:
  [<0000000094c02615>] __alloc_skb+0x5e/0x250 net/core/skbuff.c:198
  [<00000000e5386cbd>] alloc_skb include/linux/skbuff.h:1083 [inline]
  [<00000000e5386cbd>] kcm_sendmsg+0x3b6/0xa50 net/kcm/kcmsock.c:967 [1]
  [<00000000f1613a8a>] sock_sendmsg_nosec net/socket.c:652 [inline]
  [<00000000f1613a8a>] sock_sendmsg+0x4c/0x60 net/socket.c:672

Reported-and-tested-by: syzbot+b039f5699bd82e1fb011@syzkaller.appspotmail.com
Fixes: ab7ac4eb ("kcm: Kernel Connection Multiplexor module")
Cc: stable@vger.kernel.org
Signed-off-by: NPavel Skripkin <paskripkin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/kcm/kcmsock.c

@@ -1066,6 +1066,11 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 		goto partial_message;
 	}
 
+	if (skb_has_frag_list(head)) {
+		kfree_skb_list(skb_shinfo(head)->frag_list);
+		skb_shinfo(head)->frag_list = NULL;
+	}
+
 	if (head != kcm->seq_skb)
 		kfree_skb(head);