powerpc/64/syscall: Zero volatile registers when returning

Kernel addresses and potentially other sensitive data could be leaked
in volatile registers after a syscall.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200225173541.1549955-27-npiggin@gmail.com