xfrm: Fix infinite loop in xfrm_get_dst_nexthop with transport mode.

On transport mode we forget to fetch the child dst_entry
before we continue the while loop, this leads to an infinite
loop. Fix this by fetching the child dst_entry before we
continue the while loop.

Fixes: 0f6c480f23f4 ("xfrm: Move dst->path into struct xfrm_dst")
Reported-by: syzbot+7d03c810e50aaedef98a@syzkaller.appspotmail.com
Tested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -2732,14 +2732,14 @@ static const void *xfrm_get_dst_nexthop(const struct dst_entry *dst,
 	while (dst->xfrm) {
 		const struct xfrm_state *xfrm = dst->xfrm;
 
+		dst = xfrm_dst_child(dst);
+
 		if (xfrm->props.mode == XFRM_MODE_TRANSPORT)
 			continue;
 		if (xfrm->type->flags & XFRM_TYPE_REMOTE_COADDR)
 			daddr = xfrm->coaddr;
 		else if (!(xfrm->type->flags & XFRM_TYPE_LOCAL_COADDR))
 			daddr = &xfrm->id.daddr;
-
-		dst = xfrm_dst_child(dst);
 	}
 	return daddr;
 }