ksmbd: smbd: fix kernel oops during server shutdown

if server shutdown happens in the situation that
there are connections, workqueue could be destroyed
before queueing disconnect work.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>