netlink: Fully clear parser state between messages

Failing to reset the cookie between messages can lead to an attempt
to interpret a zeroed buffer as a struct nlattr, causing a length
calculation to underflow, resulting in a memcpy() call where the
length exceeds the actual size of the buffer.

MFC after:	1 week
PR:		283797
Reviewed by:	glebius
Differential Revision:	https://reviews.freebsd.org/D51634

sys/netlink/netlink_io.c

@@ -308,6 +308,7 @@ static void
 npt_clear(struct nl_pstate *npt)
 {
 	lb_clear(&npt->lb);
+	npt->cookie = NULL;
 	npt->error = 0;
 	npt->err_msg = NULL;
 	npt->err_off = 0;