libbpf: Use static const fmt string in __bpf_printk

The __bpf_printk convenience macro was using a 'char' fmt string holder
as it predates support for globals in libbpf. Move to more efficient
'static const char', but provide a fallback to the old way via
BPF_NO_GLOBAL_DATA so users on old kernels can still use the macro.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210917182911.2426606-6-davemarchevsky@fb.com

tools/lib/bpf/bpf_helpers.h

@@ -216,9 +216,15 @@ enum libbpf_tristate {
 		     ___param, sizeof(___param));		\
 })
 
+#ifdef BPF_NO_GLOBAL_DATA
+#define BPF_PRINTK_FMT_MOD
+#else
+#define BPF_PRINTK_FMT_MOD static const
+#endif
+
 #define __bpf_printk(fmt, ...)				\
 ({							\
-	char ____fmt[] = fmt;				\
+	BPF_PRINTK_FMT_MOD char ____fmt[] = fmt;	\
 	bpf_trace_printk(____fmt, sizeof(____fmt),	\
 			 ##__VA_ARGS__);		\
 })