aio: fix possible invalid memory access when DEBUG is enabled

commit 91d80a84bbc8f28375cca7e65ec666577b4209ad upstream.

dprintk() shouldn't access @ring after it's unmapped.

Signed-off-by: Zhao Hongjiang <zhaohongjiang@huawei.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>