find double declaration of global variables in sparse check

We do not initialise global variables to 0 or NULL and it allows GCC
to declare them multiple times in different places.  This detects the
unsafe usage when doing sparse check.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
fix double declaration of global variables

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>