powerpc/fault: Unnest definition of page_fault_is_write() and page_fault_is_bad()

To make it more readable, separate page_fault_is_write() and page_fault_is_bad()
to avoir several levels of #ifdefs

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6afaac2495248d68f94c438c5ec36b6010931de5.1607491748.git.christophe.leroy@csgroup.eu