commit: get the overlay node before manipulating the backing chain

commit: get the overlay node before manipulating the backing chain

The 'block-commit' command has a 'top' parameter to specify the
topmost node from which the data is going to be copied.

   [E] <- [D] <- [C] <- [B] <- [A]

In this case if [C] is the top node then this is the result:

   [E] <- [B] <- [A]

[B] must be modified so its backing image string points to [E] instead
of [C]. commit_start() takes care of reopening [B] in read-write
mode, and commit_complete() puts it back in read-only mode once the
operation has finished.

In order to find [B] (the overlay node) we look for the node that has
[C] (the top node) as its backing image. However in commit_complete()
we're doing it after [C] has been removed from the chain, so [B] is
never found and remains in read-write mode.

This patch gets the overlay node before the backing chain is
manipulated.

Signed-off-by: Alberto Garcia <[email protected]>
Message-id: 1471836963[email protected]
Signed-off-by: Max Reitz <[email protected]>