nvdimm/namespace: drop unneeded temporary variable in size_store()

Refactor size_store() in order to remove temporary variable on stack
by joining conditionals.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220607153750.33639-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>