Usually, after g_pointer_bit_lock() we want to read the pointer that we
have. In many cases, when we g_pointer_bit_lock() a pointer, we can
access it afterwards without atomic, as nobody is going to modify the
pointer then.

However, gdataset also supports g_datalist_set_flags(), so the pointer
may change at any time and we must always use atomics to read it. For
that reason, g_datalist_lock_and_get() does an atomic read right after
g_pointer_bit_lock().

g_pointer_bit_lock() can easily access the value that it just set. Add
g_pointer_bit_lock_and_get() which can return the value that gets set
afterwards.

Aside from saving the second atomic-get in certain scenarios, the
returned value is also atomically the one that we just set.