vm: use func for the correct function name

In fee2a2fa39834d8d5eaa981298fce9d2ed31546d the KASSERTs in
vm_page_unwire_noq() changed from "vm_page_unwire" to "vm_page_unref".
While the former no longer was part of that function the latter does
not exist as a function and is highly confusing when hit when using
tools to lookup the functions and not doing a full-text search.
Use %s func for printing the function name, as that will do the
right thing as code moves around and functions get renamed.

Hit: while debugging a wired page leak with linuxkpi/iwlwifi
Sponsored by: The FreeBSD Foundation
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D31635