ring-buffer: make moving the tail page a separate function

Ingo Molnar thought the code would be cleaner if we used a function call
instead of a goto for moving the tail page. After implementing this,
it seems that gcc still inlines the result and the output is pretty much
the same. Since this is considered a cleaner approach, might as well
implement it.

[ Impact: code clean up ]

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