Make net.fibs growable.

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