ip_mroute: do not call epoch_waitwhen lock is taken

mrouter_done is called with RAW IP lock taken. Some annoying
printfs are visible on the console if INVARIANTS option is enabled.

Provide atomic-based mechanism which counts enters and exits from/to
critical section in ip_input and ip_output.
Before de-initialization of function pointers ensure (with busy-wait)
that mrouter de-initialization is visible to all readers and that we don't
remove pointers (like ip_mforward etc.) in the middle of packet processing.