Enter epoch when addding IPv4 multicast forwarding cache entry

[karels]

Summary

The code path from the IPv4 multicast setsockopt could call ip_output()
without entering an epoch. Specifically, the MRT_ADD_MFC setbsocopt
would call add_mfc(), which in turn called ip_mdq() to send queued
packets. This resulted in an epoch assert failure in ip_output().
Enter an epoch in add_mfc(), and add some epoch asserts to check
for similar failures.

Test Plan

tested with mrouted and rwhod -m

[bz]

Sounds reasonable to me and I like the extra asserts for documentation purposes; I always wonder if the entire ioctl paths should just acquire/release epoch but that's a totally different can of worms.