ksmbd: mark SMB2_SESSION_EXPIRED to session when destroying previous session

Currently ksmbd exit connection as well destroying previous session.
When testing durable handle feaure, I found that
destroy_previous_session() should destroy only session, i.e. the
connection should be still alive. This patch mark SMB2_SESSION_EXPIRED
on the previous session to be destroyed later and not used anymore.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>