block: make BlockLimits::max_pdiscard 64bit

block: make BlockLimits::max_pdiscard 64bit

We are going to support 64 bit discard requests. Now update the
limit variable. It's absolutely safe. The variable is set in some
drivers, and used in bdrv_co_pdiscard().

Update also max_pdiscard variable in bdrv_co_pdiscard(), so that
bdrv_co_pdiscard() is now prepared for 64bit requests. The remaining
logic including num, offset and bytes variables is already
supporting 64bit requests.

So the only thing that prevents 64 bit requests is limiting
max_pdiscard variable to INT_MAX in bdrv_co_pdiscard().
We'll drop this limitation after updating all block drivers.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210903102807.27127-10-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>