net/iflib.c: move out scheduler-depended code into the hook
Add sched_find_l2_neighbor(). This really should be not
scheduler-depended, in does not have anything to do with scheduler at
all. But for now keep the same code structure.
Reviewed by: olce
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D54831