libnvdimm: use devm_add_action_or_reset()

Clean up needless calls to the action routine by letting
devm_add_action_or_reset() call it automatically.  This does cause the
disk to registered and immediately unregistered when a memory allocation
fails, but the block layer should be prepared for such an event.

Reported-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>