if_enc(4): Use new KPI to create enc interface

[zlei]

Summary

This driver only allow exactly one instance to be created. Clone
creating additional interfaces will get error EEXIST which is somewhat
confusing.

Convert to new KPI so we can get less confusing error ENOSPC.

MFC after: 1 week

Test Plan

Before the change:

# ifconfig enc create
ifconfig: interface enc already exists
# ifconfig enc1 create
ifconfig: interface enc1 already exists

After the change:

ifconfig enc create
ifconfig: SIOCIFCREATE2 (enc): No space left on device
# ifconfig enc1 create
ifconfig: SIOCIFCREATE2 (enc1): No space left on device

[zlei]

Rebased to latest main.