No caller was using the flags argument, and this function is internal
only, so we might as well skip it.

* src/util/util.h (safezero): Update signature.
* src/util/util.c (safezero): Update function.
* src/locking/lock_driver_sanlock.c
(virLockManagerSanlockSetupLockspace)
(virLockManagerSanlockCreateLease): Update all callers.
* src/storage/storage_backend.c (createRawFile): Likewise.