dm writecache: handle DAX to partitions on persistent memory correctly

The function dax_direct_access doesn't take partitions into account,
it always maps pages from the beginning of the device. Therefore,
persistent_memory_claim() must get the partition offset using
get_start_sect() and add it to the page offsets passed to
dax_direct_access().

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: 48debafe ("dm: add writecache target")
Cc: stable@vger.kernel.org # 4.18+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>