Avoid duplicated calls to pmap_page_get_memattr().

Avoid logging inconsistency for the /dev/mem device at all. The driver
leaves memattr intact, and the corrective action in the device pager
handles it right.

In the logged warning, name the driver we blame, and show memory
attributes values.

Reported and tested by: pho
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D6149