mips: Use PAGE_SIZE for INIT_TASK_DATA alignment again

957b369c (mips: Use generic init_task) optimistically replaced the
PAGE_SIZE INIT_TASK_DATA alignment with THREAD_SIZE, but THREAD_SIZE
is not defined, so the linking stage breaks.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>