In the 64-bit PTE case there's no point in restricting the encoding
to the low bits of the PTE, we can instead bump all of this up to
the high 32 bits and extend PTE_FILE_MAX_BITS to 32, adopting the
same convention used by x86 PAE.

There's a minor discrepency between the number of bits used for the
swap type encoding between 32 and 64-bit PTEs, but this is unlikely
to cause any problem given the extended offset.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>