dm snap: add fake origin_direct_access

dax-capable mapped-device is marked as DM_TYPE_DAX_BIO_BASED,
which supports both dax and bio-based operations.  dm-snap
needs to work with dax-capable device when bio-based operation
is used.

Add fake origin_direct_access() to origin device so that its
origin device is also marked as DM_TYPE_DAX_BIO_BASED for
dax-capable device.  This allows to extend target's DM table.
dm-snap works normally when bio-based operation is used.

dm-snap does not support dax operation, and mount with dax
option to a target device or snapshot device fails.

Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>