With more excessive use of modules, more kernel parts working with
VIMAGE, and feature richness and global state increasing the 8k of
vnet module space are no longer sufficient for people and loading
multiple modules, e.g., pf(4) and ipl(4) or ipsec(4) will fail on
the second module.

Increase the module space to 8 * PAGE_SIZE which should be enough
to hold multiple firewalls, ipsec, multicast (as in the old days was
a problem), epair, carp, and any kind of other vnet enabled modules.

Sadly this is a global byte array part of the vnet_set, so we cannot
dynamically change its size;  otherwise a TUNABLE would have been
a better solution.