ksmbd: fix multichannel connection failure

ksmbd check that the session of second channel is in the session list of
first connection. If it is in session list, multichannel connection
should not be allowed.

Fixes: b9562943 ("ksmbd: fix racy issue from session lookup and expire")
Reported-by: Sean Heelan <seanheelan@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>