Track device's NUMA domain in ifnet & alloc ifnet from NUMA local memory

This commit adds new if_alloc_domain() and if_alloc_dev() methods to
allocate ifnets. When called with a domain on a NUMA machine,
ifalloc_domain() will record the NUMA domain in the ifnet, and it will
allocate the ifnet struct from memory which is local to that NUMA
node. Similarly, if_alloc_dev() is a wrapper for if_alloc_domain
which uses a driver supplied device_t to call ifalloc_domain() with
the appropriate domain.

Note that the new if_numa_domain field fits in an alignment pad in
struct ifnet, and so does not alter the size of the structure.

Reviewed by: glebius, kib, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19930