ring-buffer: fix free page

The pages of a buffer was originally pointing to the page struct, it
now points to the page address. The freeing of the page still uses
the page frame free "__free_page" instead of the correct free_page to
the address.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>