MIPS: Fix 64k page support for 32 bit kernels.

TASK_SIZE was defined as 0x7fff8000UL which for 64k pages is not a
multiple of the page size.  Somewhere further down the math fails
such that executing an ELF binary fails.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Tested-by: Joshua Henderson <joshua.henderson@microchip.com>