nvdimm: rectify the illogical code within nd_dax_probe()

nvdimm: rectify the illogical code within nd_dax_probe()

When nd_dax is NULL, nd_pfn is consequently NULL as well. Nevertheless,
it is inadvisable to perform pointer arithmetic or address-taking on a
NULL pointer.
Introduce the nd_dax_devinit() function to enhance the code's logic and
improve its readability.

Signed-off-by: Yi Yang <yiyang13@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20241108085526.527957-1-yiyang13@huawei.com
Signed-off-by: Ira Weiny <ira.weiny@intel.com>