fuse: use READ_ONCE on congestion_threshold and max_background

[ Upstream commit 2a23f2b8adbe4bd584f936f7ac17a99750eed9d7 ]

Since they are of unsigned int type, it's allowed to read them
unlocked during reporting to userspace. Let's underline this fact
with READ_ONCE() macroses.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>