MFC various iflib fixes from head

Included revisions:
r347418 - iflib: use default ntxd and rxd when user value is not power of 2
r348372 - iflib: provide probe wrapper for vendor drivers
r350306 - iflib: fix dangling device softc pointer
r350507 - iflib: remove kobject class reference increment
r350509 - iflib: Prevent kernel panic caused by loading driver with a specific interrupt configuration
r351152 - iflib: add iflib_deregister to help cleanup on exit

Sponsored by: Intel Corporation