perf tools: No need to unconditionally read the max_stack sysctls

Let tools that need to have those variables with the sysctl current
values use a function that will read them.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-1ljj3oeo5kpt2n1icfd9vowe@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/util.h

@@ -45,6 +45,8 @@ int hex2u64(const char *ptr, u64 *val);
 extern unsigned int page_size;
 int __pure cacheline_size(void);
 
+int sysctl__max_stack(void);
+
 int fetch_kernel_version(unsigned int *puint,
 			 char *str, size_t str_sz);
 #define KVER_VERSION(x)		(((x) >> 16) & 0xff)