ring-buffer: Move access to commit_page up into function used

With the change of the way we process commits. Where a commit only happens
at the outer most level, and that we don't need to worry about
a commit ending after the rb_start_commit() has been called, the code
use to grab the commit page before the tail page to prevent a possible
race. But this race no longer exists with the rb_start_commit()
rb_end_commit() interface.

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