dm vdo: add 'log_level' module parameter

Expose control over dm-vdo's log-level in terms of a module param. It
can be read and written via /sys/module/dm_vdo/parameters/log_level.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>