ifnet: merge ifindex_alloc(), ifnet_setbyindex(), if_grow() and call magic

Now it is possible to just merge all this complexity into single
linear function. Note that IFNET_WLOCK() is a sleepable lock, so
we can M_WAITOK and epoch_wait_preempt().

Reviewed by: melifaro, bz, kp
Differential revision: https://reviews.freebsd.org/D33262