MINOR: ring: clarify the usage of ring_size() and add ring_allocated_size()

MINOR: ring: clarify the usage of ring_size() and add ring_allocated_size()

There's currently an abiguity around ring_size(), it's said to return
the allocated size but returns the usable size. We can't change it as
it's used everywhere in the code like this. Let's fix the comment and
add ring_allocated_size() instead for anything related to allocation.