pf: Fix missing connection rate for DIOCGETSRCNODES

The function pf_src_node_copy() copies struct pf_ksrc_node to its ioctl
counterpart. It was missing copying of the conn_rate field, even though
the code for adjusting the output for pfctl was in place.

Add copying of conn_rate.

Reviewed by: kp
Approved by: kp (mentor)
MFC after: 2 weeks
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D47679