cifsd: add the check to prevent potential overflow with smb_strtoUTF16() and UNICODE_LEN()

Add the check to prevent potential overflow with smb_strtoUTF16() and
UNICODE_LEN().

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>