ring-buffer: Use rb_page_size() instead of open coded head_page size

ring-buffer: Use rb_page_size() instead of open coded head_page size

There's a helper function to get a ring buffer page size (the number
of bytes of data recorded on the page), called rb_page_size().
Use that instead of open coding it.

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