nvdimm: Clean up __nd_ioctl() and remove gotos

Utilize scoped based resource management to clean up the code and
and remove gotos for the __nd_ioctl() function.

Change allocation of 'buf' to use kvzalloc() in order to use
vmalloc() memory when needed and also zero out the allocated
memory.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>