powerpc/iseries: use DECLARE_COMPLETION_ONSTACK for non-constant completion

The _ONSTACK variant should be used for on-stack completion,
otherwise it will break lockdep.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>