Use thread-safe function pointer in libbpf_print

This patch fixes a thread safety bug where libbpf_print uses the
global variable storing the print function pointer rather than the local
variable that had the print function set via __atomic_load_n.

Fixes: f1cb927cdb62 ("libbpf: Ensure print callback usage is thread-safe")
Signed-off-by: Jonathan Wiepert <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Mykyta Yatsenko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/libbpf.c

···
 	old_errno = errno;
 
 	va_start(args, format);
-	__libbpf_pr(level, format, args);
+	print_fn(level, format, args);
 	va_end(args);
 
 	errno = old_errno;