Existense of PCB route caching doesn't allow us to use new fast route
lookup KPI in ip_output() like it is already used in ip_forward().
However, when there is no PCB provided we can use fast KPI, gaining
performance advantage.

Typical case when ip_output() is called without a PCB pointer is a
sendto(2) on a not connected UDP socket. In practice DNS servers do
this.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D19804