block: introduce preallocate filter

block: introduce preallocate filter

It's intended to be inserted between format and protocol nodes to
preallocate additional space (expanding protocol file) on writes
crossing EOF. It improves performance for file-systems with slow
allocation.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201021145859.11201-9-vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
[mreitz: Two comment fixes, and bumped the version from 5.2 to 6.0]
Signed-off-by: Max Reitz <mreitz@redhat.com>