ring-buffer: remove obsolete comment for free_buffer_page()

The comment refers to mm/slob.c which is being removed. It comes from
commit ed56829 ("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 e4c2ce8 ("ring_buffer:
allocate buffer page pointer"). Thus the comment is not accurate anyway,
remove it.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: Ingo Molnar <[email protected]>
Reported-by: Mike Rapoport <[email protected]>
Suggested-by: Steven Rostedt (Google) <[email protected]>
Fixes: e4c2ce8 ("ring_buffer: allocate buffer page pointer")
Signed-off-by: Vlastimil Babka <[email protected]>
Reviewed-by: Mukesh Ojha <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

kernel/trace/ring_buffer.c

@@ -354,10 +354,6 @@ static void rb_init_page(struct buffer_data_page *bpage)
 	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);