libnvdimm, pmem: fix size trim in pmem_direct_access()

This masking prevents access to the end of the device via dax_do_io(),
and is unnecessary as arch_add_memory() would have rejected an unaligned
allocation.

Cc: <stable@vger.kernel.org>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>