powerpc: rearrange do_page_fault error case to be inside exception_enter

This keeps the context tracking over the entire interrupt handler which
helps later with moving context tracking into interrupt wrappers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-14-npiggin@gmail.com