btrfs: move delayed ref flushing for qgroup into qgroup helper

The commit d6726335 ("btrfs: qgroup: Make snapshot accounting work
with new extent-oriented qgroup.") added a flush of the delayed refs
during snapshot creation in order to get the qgroup accounting properly.
However this code has changed and been moved to it's own helper that is
skipped if qgroups are turned off.  Move the flushing to the helper, as
we do not need it when qgroups are turned off.

Also add a comment explaining why it exists, and why it doesn't actually
save us.  This will be helpful later when we try to fix qgroup
accounting properly.

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