device-dax: avoid hang on error before devm_memremap_pages()

dax_pmem_percpu_exit() waits for dax_pmem_percpu_release() to invoke the
dax_pmem->cmp completion.  Unfortunately this approach to cleaning up
the percpu_ref only works after devm_memremap_pages() was successful.

If devm_add_action_or_reset() or devm_memremap_pages() fails,
dax_pmem_percpu_release() is not invoked.  Therefore
dax_pmem_percpu_exit() hangs waiting for the completion:

  rc = devm_add_action_or_reset(dev, dax_pmem_percpu_exit,
  				&dax_pmem->ref);
  if (rc)
  	return rc;

  dax_pmem->pgmap.ref = &dax_pmem->ref;
  addr = devm_memremap_pages(dev, &dax_pmem->pgmap);

Avoid the hang by calling percpu_ref_exit() in the error paths instead
of going through dax_pmem_percpu_exit().

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>