to find and unlink routes that reference the detached ifnet: make
if_rt_walktree() return ERESTART whenever it has deleted a route.
Whenever rt_walktree() returns ERESTART, if_detach() restarts it.
I believe that this fix resembles one by Jonathan Kollasch or by someone
else, which has languished in a PR for too long. Sorry!
Tested by me and by Jeff Rizzo.
XXX It's supposed to be safe for rn_walktree() to apply to the routing
XXX table a routine that may delete routes. Why isn't it safe in
XXX practice?
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