replace microtime with getmicrouptime in ip_mroute.c

[David Gwynne]

it uses the time to rate limit the sending of assertion messages.

there are two reasons for this change.

firstly, using uptime instead of wall time means the intervals will
be measured against a a monotonic clock that isnt skewed by clock
changes. it should therefore always be rate limited to 30 seconds,
even if the clock jumps in the middle of that interval.

secondly, using the get variant of the api means it can use a cached clock time 
at the expense of accuracy. accuracy between 30s intervals isnt necessary in 
this use case, but making it use the faster clock read is good cos this path is 
in response to packets, so it will mitigate a DoS.

ok?

after this i intend to tweak the code to use ratecheck().

Index: ip_mroute.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_mroute.c,v
retrieving revision 1.90
diff -u -p -r1.90 ip_mroute.c
--- ip_mroute.c 7 Mar 2016 18:44:00 -0000       1.90
+++ ip_mroute.c 8 Sep 2016 01:49:05 -0000
@@ -1482,7 +1482,7 @@ ip_mdq(struct mbuf *m, struct ifnet *ifp
                                return (0);
                        }
 
-                       microtime(&now);
+                       getmicrouptime(&now);
 
                        TV_DELTA(rt->mfc_last_assert, now, delta);