md/dm-writecache: Don't request pointer dummy_addr when not required

Function persistent_memory_claim doesn't need to get local pointer
dummy_addr from direct_access. Using NULL instead of having to pass
in a useless local pointer that caller then just throw away.

Suggested-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>