Remove remaining mentions of pr_usrreq.

When struct pr_usrreq was folded into struct protosw and the function
pointers it contained were renamed from pru_* to pr_* in 2022, a
number of references to the old names in comments and error messages
were missed. Chase them down and fix them.

Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: kevans, glebius
Differential Revision: https://reviews.freebsd.org/D50190