ksmbd: Prevent integer overflow in calculation of deadtime

The user can set any value for 'deadtime'. This affects the arithmetic
expression 'req->deadtime * SMB_ECHO_INTERVAL', which is subject to
overflow. The added check makes the server behavior more predictable.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 0626e664 ("cifsd: add server handler for central processing and tranport layers")
Cc: stable@vger.kernel.org
Signed-off-by: Denis Arefev <arefev@swemel.ru>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>