block/block-copy: reduce intersecting request lock

block/block-copy: reduce intersecting request lock

Currently, block_copy operation lock the whole requested region. But
there is no reason to lock clusters, which are already copied, it will
disturb other parallel block_copy requests for no reason.

Let's instead do the following:

Lock only sub-region, which we are going to operate on. Then, after
copying all dirty sub-regions, we should wait for intersecting
requests block-copy, if they failed, we should retry these new dirty
clusters.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Andrey Shinkevich <[email protected]>
Message-Id: <20200311103004[email protected]>
Signed-off-by: Max Reitz <[email protected]>