ceph: fix NULL pointer dereference for req->r_session

ceph: fix NULL pointer dereference for req->r_session

[ Upstream commit 5bd76b8de5b74fa941a6eafee87728a0fe072267 ]

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>