netlink: add forgotten pru_rcvd handler.

There were changes in -HEAD domain/protosw setup logic and
.pru_rcvd netlink handler was missed when performing the merge.
Lack of this handler resulted in userland being waiting forever
when performing large dumps of data.

This change restores the handler as direct commit to stable/13.

Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>