ring-buffer: remove obsolete comment for free_buffer_page()

The comment refers to mm/slob.c which is being removed. It comes from
commit ed56829cb319 ("ring_buffer: reset buffer page when freeing") and
according to Steven the borrowed code was a page mapcount and mapping
reset, which was later removed by commit e4c2ce82ca27 ("ring_buffer:
allocate buffer page pointer"). Thus the comment is not accurate anyway,
remove it.

Link: https://lore.kernel.org/linux-trace-kernel/[e-mail]

Cc: Masami Hiramatsu <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Reported-by: Mike Rapoport <[e-mail]>
Suggested-by: Steven Rostedt (Google) <[e-mail]>
Fixes: e4c2ce82ca27 ("ring_buffer: allocate buffer page pointer")
Signed-off-by: Vlastimil Babka <[e-mail]>
Reviewed-by: Mukesh Ojha <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

kernel/trace/ring_buffer.c

···
 	local_set(&bpage->commit, 0);
 }
 
-/*
- * Also stolen from mm/slob.c. Thanks to Mathieu Desnoyers for pointing
- * this issue out.
- */
 static void free_buffer_page(struct buffer_page *bpage)
 {
 	free_page((unsigned long)bpage->page);