Any found to be using the algorithm that is about to go away are switched back
to NewReno to avoid leaving dangling pointers which would trigger a panic. For
VIMAGE kernels, there is a list per vnet to walk, yet the implementation was
only examining one of the vnet lists.

Fix the implementation of the above feature for VIMAGE kernels by looping
through all active TCP control blocks across all vnets.

Sponsored by:	FreeBSD Foundation
Tested by:	Mikolaj Golub <to.my.trociny at gmail com>
Reviewed by:	bz (briefly)
MFC after:	11 weeks