Don't call kresolv_list() if using netstat on live kernel

kresolve_list() is calling many kldsym(2). Removing that call on when collecting
stats for the running kernel improves the startup time and CPU usage.

Submitted by: Nikita Kozlov (nikita.kozlov@blade-group.com)
Reviewed by: cem
MFC after: 2 weeks
Sponsored by: blade
Differential Revision: https://reviews.freebsd.org/D12151