dns_client: directly initialize g_dns_servers and remove dns_initialize directly

[zhhyu7]

Summary

g_dns_servers need init before dns_query, Otherwise sim can not add default dns server when use usrsock mode to share host network.

Avoid similar problems in the future, so directly initialize g_dns_servers.

Impact

Fix the regression from: https://github.com/apache/incubator-nuttx/pull/6770

Testing

internal testing