Reduce the vnet_set module size of ip_mroute to allow loading as a module.

With VIMAGE kernels modules get special treatment as they need
to also keep the original values and make copies for each instance.
For that a few pages of vnet modspace are provided and the
kernel-linker and the VNET framework know how to deal with things.
When the modspace is (almost) full, other modules which would
overflow the modspace cannot be loaded and kldload will fail.

ip_mroute uses a lot of variable space, mostly be four big arrays:
set_vnet 0000000000000510 vnet_entry_multicast_register_if
set_vnet 0000000000000700 vnet_entry_viftable
set_vnet 0000000000002000 vnet_entry_bw_meter_timers
set_vnet 0000000000002800 vnet_entry_bw_upcalls

Dynamically malloc the three big ones for each instance we need
and free them again on vnet teardown (the 4th is an ifnet).
That way they only need module space for a single pointer and
allow a lot more modules using virtualized variables to be loaded
on a VNET kernel.

PR: 206583
Reviewed by: hselasky, kp
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D22443