powerpc/book3e: Fix extlb size

The calculation of the size for the exception save area of the TLB
miss handler is wrong, luckily it's too big not too small.

Rework it to make it a bit clearer, and also correct. We want 3 save
areas, each EX_TLB_SIZE _bytes_.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>