perf tests api-io: Use zfree() to reduce chances of use after free

Do defensive programming by using zfree() to initialize freed pointers
to NULL, so that eventual use after free result in a NULL pointer deref
instead of more subtle behaviour.

Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/api-io.c

···
 #include "tests.h"
 #include <api/io.h>
 #include <linux/kernel.h>
+#include <linux/zalloc.h>
 
 #define TEMPL "/tmp/perf-test-XXXXXX"
 
···
 
 static void cleanup_test(char path[PATH_MAX], struct io *io)
 {
-	free(io->buf);
+	zfree(&io->buf);
 	close(io->fd);
 	unlink(path);
 }