Now that PAGE_SIZE is available to assembly directly, there is no need
to separately expose it as _PAGE_SIZE through asm-offsets.

In addition, remove _PAGE_SHIFT from asm-offsets, since it was never
needed, and is not used anywhere.
Signed-off-by: NNelson Elhage <nelhage@ksplice.com>
Signed-off-by: NTim Abbott <tabbott@ksplice.com>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>