Enter epoch when addding IPv4 multicast forwarding cache entry

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.

Reviewed by: kp, bz, wma, cy
Differential Revision: https://reviews.freebsd.org/D34624