tools: bpftool: clean up the JSON writer before exiting in usage()

The writer is cleaned at the end of the main function, but not if the
program exits sooner in usage(). Let's keep it clean and destroy the
writer before exiting.

Destruction and actual call to exit() are moved to another function so
that clean exit can also be performed without printing usage() hints.

Fixes: d35efba99d ("tools: bpftool: introduce --json and --pretty options")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/main.h

@@ -41,6 +41,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <linux/bpf.h>
+#include <linux/compiler.h>
 #include <linux/kernel.h>
 #include <linux/hashtable.h>
 
@@ -80,7 +81,7 @@ void p_info(const char *fmt, ...);
 
 bool is_prefix(const char *pfx, const char *str);
 void fprint_hex(FILE *f, void *arg, unsigned int n, const char *sep);
-void usage(void) __attribute__((noreturn));
+void usage(void) __noreturn;
 
 struct pinned_obj_table {
 	DECLARE_HASHTABLE(table, 16);