powerpc/32s: Reorganise DSI handler.

The part decidated to handling hash_page() is fully unneeded for
processors not having real hash pages like the 603.

Lets enlarge the content of the feature fixup, and provide
an alternative which jumps directly instead of getting NIPs.

Also, in preparation of VMAP stacks, the end of DSI handler has moved
to later in the code as it won't fit anymore once VMAP stacks
are there.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c31b22c91af8b011d0a4fd9e52ad6afb4b593f71.1576916812.git.christophe.leroy@c-s.fr