dm mpath: move trigger_event member to the end of 'struct multipath'

Allows the 'work_mutex' member to no longer cross a cacheline.

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