ring-buffer: export symbols

ring-buffer: export symbols

I'm adding a module to do a series of tests on the ring buffer as well
as benchmarks. This module needs to have more of the ring buffer API
exported. There's nothing wrong with reading the ring buffer from a
module.

[ Impact: allow modules to read pages from the ring buffer ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>