ifsq: Let ifaltq_subque know its related hardware TX queue's serializer

This avoids following operations on packet transmission hot path:
- Dereferening device driver supplied serialize function pointers
- Locating hardware TX queue's serializer

Comparing to the lwkt_serialize functions, the above two operations are
costful.

Driver changes:
- For device drivers which use the default ifnet serializer, no additional
  code will be needed, if_attach() will assign ifnet serializer to
  ifaltq_subque.
- For device drivers which use independent serializers for main function,
  RX queues and TX queues, ifsq_set_hw_serialize() must be called to
  properly assign the hardware TX queue's serializer to ifaltq_subque.
  Drivers in this category are bce(4), emx(4), igb(4) and jme(4).
ifnet: Simplify ifnet_serialize_array function interface

It is no longer used to hold RX or TX serializers, so the RX/TX
serializer offset function arguments are not necessary at all.
route(8): Fix a possible infinite loop in "route flush"

Obtained-from: FreeBSD 230989
DragonFly-bug: http://bugs.dragonflybsd.org/issues/2523