net/core: Remove unused assignment operations and variable

Although if_info_size is assigned, it has not been used. And the variable
should also be deleted.

The clang_analyzer complains as follows:

net/core/rtnetlink.c:3806: warning:

Although the value stored to 'if_info_size' is used in the enclosing
expression, the value is never actually read from 'if_info_size'.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>