ring-buffer: Fix memleak in ring_buffer_free()

I noticed oprofile memleaked in linux-2.6 current tree,
and tracked this ring-buffer leak.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
LKML-Reference: <4A7C06B9.2090302@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>