Coccinelle: ifnullfree: Trim the warning reported in report mode

Remove the unncessary part of the warning reported, in the report
mode, so that a single warning produced does not exceed more than line
and hence improve readability of the warnings produced in the subsequent
reports to a file.

Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>