On CONFIG_64BIT_PHYS_ADDR, pfn always fits in 'unsigned long', but

pfn<<PAGE_SHIFT sometimes extends beyond.  The pte is big enough to hold
'long long', but the shift in pfn_pte() needs to do its calculation with
enough bits to hold the result.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>