commit: Use real permissions in commit block job

This is probably one of the most interesting conversions to the new
op blocker system because a commit block job intentionally leaves some
intermediate block nodes in the backing chain that aren't valid on their
own any more; only the whole chain together results in a valid view.

In order to provide the 'consistent read' permission to the parents of
the 'top' node of the commit job, a new filter block driver is inserted
above 'top' which doesn't require 'consistent read' on its backing
chain. Subsequently, the commit job can block 'consistent read' on all
intermediate nodes without causing a conflict.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
