This allows the caller to take the lock on the GData first, and perform
some operations.
This is useful under the assumption, that the caller can find cases
where calling g_datalist_id_update_atomic() is unnecessary, but it's
still necessary to perform that check while holding a lock.
That will yield better performance, if we can avoid calling
g_datalist_id_update_atomic(). That matters for checking the
toggle-notify in g_object_ref()/g_object_unref().
Note that with "already_locked", g_datalist_id_update_atomic() will
still unlock the GData at the end. That is because the API of
g_datalist_id_update_atomic() requires that it might re-allocate the
buffer, and it can do a more efficient unlock in that case, instead of
leaving it to the caller. The usage and purpose of this parameter is
very special, so this asymmetry is taken because the only callers will
be fine with this behavior, and it results in potentially more efficient
unlocking.
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