if: guard against if_ioctl being NULL

There are situations where an struct ifnet has a NULL if_ioctl pointer.

For example, e6000sw creates such struct ifnets for each of its ports so it can
call into the MII code.

If there is then a link state event this calls do_link_state_change()
-> rtnl_handle_ifevent() -> dump_iface() -> get_operstate() ->
get_operstate_ether(). That wants to know if the link is up or down, so it tries
to ioctl(SIOCGIFMEDIA), which doesn't go well if if_ioctl is NULL.

Guard against this, and return EOPNOTSUPP.

PR:		275920
MFC ater:	3 days
Approved by:	re (cperciva)
Sponsored by:   Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 43387b4e574043b78a58c8bcb7575161b055fce1)
(cherry picked from commit 9a8a26aefb)

sys/net/if.c

@@ -4873,6 +4873,9 @@ if_resolvemulti(if_t ifp, struct sockaddr **srcs, struct sockaddr *dst)
 int
 if_ioctl(if_t ifp, u_long cmd, void *data)
 {
+	if (ifp->if_ioctl == NULL)
+		return (EOPNOTSUPP);
+
 	return (ifp->if_ioctl(ifp, cmd, data));
 }