dm mpath: switch to using bitops for state flags

Mechanical change that doesn't make any real effort to reduce the use of
m->lock; that will come later (once atomics are used for counters, etc).

Suggested-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Tested-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>