crimson/os/seastore: fix ceph_assert in segment_manager.h

crimson/os/seastore: fix ceph_assert in segment_manager.h

Assert is seen when crimson-osd is compiled with -DWITH_ZNS=ON and
crimson-osd is started with a regular SSD.

ceph/src/crimson/os/seastore/segment_manager.h:77 : In function 'void crimson::os::seastore::block_sm_superblock_t::validate() const', ceph_assert(%s)
get_default_backend_of_device(config.spec.dtype) == backend_type_t::SEGMENTED

The device type "dtype" is not getting propogated properly
to BlockSegmentManager instantiation causing the assert at a
later point.

Signed-off-by: Aravind Ramesh <aravind.ramesh@wdc.com>