ring-buffer: make the buffer a true circular link list

This patch changes the ring buffer data pages from using a link list
head pointer, to making each buffer page point to another buffer page
and never back to a "head".

This makes the handling of the ring buffer less complex, since the
traversing of the ring buffer pages no longer needs to account for the
head pointer.

This change also is needed to make the ring buffer lockless.

[
  Changes in version 2:

  - Added change that Lai Jiangshan mentioned.

  From: Lai Jiangshan <laijs@cn.fujitsu.com>
  Date: Thu, 11 Jun 2009 11:25:48 +0800
  LKML-Reference: <4A30793C.6090208@cn.fujitsu.com>

  I'm not sure whether these 4 lines:
	bpage = list_entry(pages.next, struct buffer_page, list);
	list_del_init(&bpage->list);
	cpu_buffer->pages = &bpage->list;

	list_splice(&pages, cpu_buffer->pages);
  equal to these 2 lines:
 	cpu_buffer->pages = pages.next;
 	list_del(&pages);

  If there are equivalent, I think the second one
  are simpler. It may be not a really necessarily cleanup.

  What I asked is: if there are equivalent, could you use these two line:
 	cpu_buffer->pages = pages.next;
	list_del(&pages);
]

[ Impact: simplify the ring buffer to help make it lockless ]

Signed-off-by: Steven Rostedt <srostedt@redhat.com>