net: sockets: getaddrinfo: use k_ variants for memory allocation

The rest of the network (and generally Zephyr subsystem code) use the k_
prefixed variants for memory allocation, which results in data goin in
the _system_heap rather than the stdlib z_malloc_heap.

The code in getaddrinfo use the z_ ones as well apart from the actual
getaddrinfo implementation, which seems like an oversight, change it to
use k_calloc and k_free as well.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>