dm vdo logger: update logging to start with "device-mapper: vdo"

Stops short of actually using DM's various logging macros (e.g. DMERR,
DMINFO, etc) because VDO's logger isn't quite compatible with them.

Also switch emit_log_message_to_kernel() from open-coding printk with
log-level to using corresponding pr_ macro.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Bruce Johnston <bjohnsto@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>