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

[glebius]

Summary

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().

[bz]

sys/net/if.c

390

Do we still need these to be double pointers? We are not returning any of these values to a caller anymore.

[glebius]

sys/net/if.c

390

I think stylistically correct to keep them double pointers, since we allocate/free an array of pointers. And V_ifindex_table is of the same type and we assign V_ifindex_table to new, and assign old to V_ifindex_table.

[kp]

sys/net/if.c

393

I dislike spelling * 2 as << 1, but won't hold up the review over it.