kern/138666: [multicast] [panic] not working multicast through igmpproxy

[Bruce Simpson]

I did a quick pass over ip_mroute.c to see if I could have introduced 
any obvious errors during refactoring; didn't see anything obvious.

The backtrace which was posted points towards a trashed rte->m pointer, 
assuming it's accurate and the arguments didn't get trashed on-stack. 
The MFC lock should be held in expire_mfc(). I'll add a lock assertion 
there for now. The VIF lock shouldn't be needed in this path.