powerpc/64s/exception: reduce page fault unnecessary loads

This avoids 3 loads in the radix page fault case, 1 load in the
hash fault case, and 2 loads in the hash miss page fault case.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-37-npiggin@gmail.com