ring-buffer: Force inline of hotpath helper functions

There's several small helper functions in ring_buffer.c that are used in the
hot path. For some reason, even though they are marked inline, gcc tends not
to enforce it. Make sure these functions are always inlined.

Link: http://lkml.kernel.org/r/20161121183700.GW26852@two.firstfloor.org



Reported-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>