[PATCH] Fix for IPv4 subnet route migration

[Ryan Stone]

Currently, the FreeBSD stack gets itself into a bizarre state if you
try to change the source IP for a subnet route (yes, this implies
using two IPs on the same subnet on a system.  Yes, I'm aware this is
a Bad Idea(TM).  Unfortunately I have a customer that insists on this
configuration)

You can see the review description for all of the gory details.  IPv6
is not affected by the issue so I only had to fix IPv4.  The review is
here:

https://reviews.freebsd.org/D14547

If anybody is interested in reviewing this fix, please subscribe
yourself to the review.  Thanks!

Ryan