ksmbd: fix racy issue from session lookup and expire

Increment the session reference count within the lock for lookup to avoid
racy issue with session expire.

Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-25737
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>