qemu-io: Allow larger write zeroes under no fallback

qemu-io: Allow larger write zeroes under no fallback

When writing zeroes can fall back to a slow write, permitting an
overly large request can become an amplification denial of service
attack in triggering a large amount of work from a small request.  But
the whole point of the no fallback flag is to quickly determine if
writing an entire device to zero can be done quickly (such as when it
is already known that the device started with zero contents); in those
cases, artificially capping things at 2G in qemu-io itself doesn't
help us.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20211203231539.3900865-4-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>