powerpc/uaccess: Also perform 64 bits copies in unsafe_copy_from_user() on ppc32

Similarly to commit 5cf773fc8f37 ("powerpc/uaccess: Also perform
64 bits copies in unsafe_copy_to_user() on ppc32")

ppc32 has an efficiant 64 bits unsafe_get_user(), so also use it in
order to unroll loops more.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/308e65d9237a14e8c0e3b22919fcf0b5e5592608.1616151715.git.christophe.leroy@csgroup.eu