MIPS: Fix PAGE_MASK definition

MIPS: Fix PAGE_MASK definition

Make PAGE_MASK an unsigned long, like it is on x86, to avoid:

In file included from arch/mips/kernel/asm-offsets.c:14:0:
include/linux/mm.h: In function '__pfn_to_pfn_t':
include/linux/mm.h:1050:2: warning: left shift count >= width of type
  pfn_t pfn_t = { .val = pfn | (flags & PFN_FLAGS_MASK), };

...where PFN_FLAGS_MASK is:

#define PFN_FLAGS_MASK (~PAGE_MASK << (BITS_PER_LONG - PAGE_SHIFT))

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: ross.zwisler@linux.intel.com
Cc: hch@lst.de
Cc: linux-mips@linux-mips.org
Cc: linux-nvdimm@lists.01.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Patchwork: https://patchwork.linux-mips.org/patch/11280/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>