Fix iflib freelist state corruption

This fixes a bug in iflib freelist management that breaks the required
correspondence between freelist indexes and driver ring slots.

PR: 243126, 243392, 240628
Reported by: avg, alexandr.oleynikov@gmail.com, Harald Schmalzbauer
Reviewed by: avg, gallatin
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23943