block/file-posix: fix workaround in raw_do_pwrite_zeroes()

We should not set overlap_bytes:

1. Don't worry: it is calculated by bdrv_mark_request_serialising() and
   will be equal to or greater than bytes anyway.

2. If the request was already aligned up to some greater alignment,
   than we may break things: we reduce overlap_bytes, and further
   bdrv_mark_request_serialising() may not help, as it will not restore
   old bigger alignment.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201203222713.13507-2-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>