loop: fix concurrent lo_open/lo_release

范龙飞 reports that KASAN can report a use-after-free in __lock_acquire.
The reason is due to insufficient serialization in lo_release(), which
will continue to use the loop device even after it has decremented the
lo_refcnt to zero.

In the meantime, another process can come in, open the loop device
again as it is being shut down. Confusion ensues.

Reported-by: 范龙飞 <long7573@126.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Git-commit: ae6650163c66a7eff1acd6eb8b0f752dcfa8eba5
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: I965ddcdc2a9d27fd56a54d2886b7c3861c21b05f
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>