UDP: let udp_pcblist() support UDP and UDP-Lite

Provide the IPPROTO_UDP in the arg2 parameter of udp_pcblist() and use
this to determine the inpcbinfo. This allows the same function to be
used in an upcoming commit to provide the list of pcbs for UDP-Lite
just by providing IPPROTO_UDPLITE in the arg2 parameter.

Reviewed by: rrs
Differential Revision: https://reviews.freebsd.org/D53218

(cherry picked from commit be3c59ff508204dbab9e3616dc82ed7620f05d0c)