block/commit: utilize job_exit shim

block/commit: utilize job_exit shim

Change the manual deferment to commit_complete into the implicit
callback to job_exit, renaming commit_complete to commit_exit.

This conversion does change the timing of when job_completed is
called to after the bdrv_replace_node and bdrv_unref calls, which
could have implications for bjob->blk which will now be put down
after this cleanup.

Kevin highlights that we did not take any permissions for that backend
at job creation time, so it is safe to reorder these operations.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 20180830015734[email protected]
Reviewed-by: Jeff Cody <[email protected]>
Signed-off-by: Max Reitz <[email protected]>