nvdimm: use 'u64' for pfn flags

A recent bugfix changed pfn_t to always be 64-bit wide, but did not
change the code in pmem.c, which is now broken on 32-bit architectures
as reported by gcc:

In file included from ../drivers/nvdimm/pmem.c:28:0:
drivers/nvdimm/pmem.c: In function 'pmem_alloc':
include/linux/pfn_t.h:15:17: error: large integer implicitly truncated to unsigned type [-Werror=overflow]
 #define PFN_DEV (1ULL << (BITS_PER_LONG_LONG - 3))

This changes the intermediate pfn_flags in struct pmem_device to
be 64 bit wide as well, so they can store the flags correctly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: db78c222

 ("mm: fix pfn_t vs highmem")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>