dm: inherit QUEUE_FLAG_SG_GAPS flags from underlying queues

A DM device must inherit the QUEUE_FLAG_SG_GAPS flags from its
underlying block devices' request queues.

This fixes problems when submitting cloned requests to multipathed
devices requiring virtually contiguous buffers.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>