dm table: fix 'all_blk_mq' inconsistency when an empty table is loaded

An earlier DM multipath table could have been build ontop of underlying
devices that were all using blk-mq.  In that case, if that active
multipath table is replaced with an empty DM multipath table (that
reflects all paths have failed) then it is important that the
'all_blk_mq' state of the active table is transfered to the new empty DM
table.  Otherwise dm-rq.c:dm_old_prep_tio() will incorrectly clone a
request that isn't needed by the DM multipath target when it is to issue
IO to an underlying blk-mq device.

Fixes: e83068a5 ("dm mpath: add optional "queue_mode" feature")
Cc: stable@vger.kernel.org # 4.8+
Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Tested-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>