ceph: fix NULL pointer dereference for req->r_session

stable inclusion
from stable-v5.10.157
commit ca3a08e9d9ebda0557c3a9d316e768f4d52c2168
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7MU59

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ca3a08e9d9ebda0557c3a9d316e768f4d52c2168

--------------------------------

[ Upstream commit 5bd76b8d ]

The request's r_session maybe changed when it was forwarded or
resent. Both the forwarding and resending cases the requests will
be protected by the mdsc->mutex.

Cc: stable@vger.kernel.org
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2137955


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>