Rework the rttimer code. Instead of a global queue and a global timeout

Rework the rttimer code. Instead of a global queue and a global timeout
use a per rttimer struct timeout. On enqueue the struct rttimer belongs
to the timeout, in case the route is removed before the timer fires
cleanup based on the timeout_del() return value. If the timeout currently
running then just clear the rtt_rt pointer and let the timeout handle the
cleanup. This should hopefully fix the icmp_pmtu_timeout crashes reported
by some people.
OK bluhm@