Make net.fibs growable.

Allow to dynamically grow the amount of fibs in each vnet.

This change alters current behavior. Currently, if one defines
ROUTETABLES > 1 in the kernel config, each vnet will be created
with the number of fibs defined in the kernel config.
After this commit vnets will be created with fibs=1.

Dynamic net.fibs is not compatible with net.add_addr_allfibs.
The plan is to deprecate the latter and make
net.add_addr_allfibs=0 default behaviour.

Reviewed by: glebius
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D26062