BUG/MEDIUM: ring: invert the length check to avoid an int overflow

BUG/MEDIUM: ring: invert the length check to avoid an int overflow

Vincent Gramer reported in GH issue #3125 a case of crash on a BUG_ON()
condition in the rings. What happens is that a message that is one byte
less than the maximum ring size is emitted, and it passes all the checks,
but once inflated by the extra +1 for the refcount, it can no longer. But
the check was made based on message size compared to space left, except
that this space left can now be negative, which is a high positive for
size_t, so the check remained valid and triggered a BUG_ON() later.

Let's compute the size the other way around instead (i.e. current +
needed) since we can't have rings as large as half of the memory space
anyway, thus we have no risk of overflow on this one.

This needs to be backported to all versions supporting multi-threaded
rings (3.0 and above).

Thanks to Vincent for the easy and working reproducer.