dm mpath: push back requests instead of queueing

There is no reason why multipath needs to queue requests internally for
queue_if_no_path or pg_init; we should rather push them back onto the
request queue.

And while we're at it we can simplify the conditional statement in
map_io() to make it easier to read.

Since mpath no longer does internal queuing of I/O the table info no
longer emits the internal queue_size.  Instead it displays 1 if queuing
is being used or 0 if it is not.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>