btrfs: unify error handling for ticket flushing

Currently we handle the cleanup of errored out tickets in both the
priority flush path and the normal flushing path.  This is the same code
in both places, so just refactor so we don't duplicate the cleanup work.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>