libnvdimm: fix devm_nvdimm_memremap() error path

The internal alloc_nvdimm_map() helper might fail, particularly if the
memory region is already busy.  Report request_mem_region() failures and
check for the failure.

Reported-by: Ryan Chen <ryan.chan105@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>