ring-buffer: use internal time stamp function

The ring_buffer_time_stamp that is exported adds a little more overhead
than is needed for using it internally. This patch adds an internal
timestamp function that can be inlined (a single line function)
and used internally for the ring buffer.

[ Impact: a little less overhead to the ring buffer ]

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