[S390] Missing initialization in common i/o layer.

Previous patch that was intended to reduce stack usage within common
i/o layer didn't consider implicit memset(..., 0, ...) used with the
initializations used before.
Add these missing memsets wherever it's not obvious that the
concerned memory region is zeroed. This should give the same semantics
as before.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>