bpftool: Replace "__fallthrough" by a comment to address merge conflict

The recent support for inline annotations in control flow graphs
generated by bpftool introduced the usage of the "__fallthrough" macro
in a switch/case block in btf_dumper.c. This change went through the
bpf-next tree, but resulted in a merge conflict in linux-next, because
this macro has been renamed "fallthrough" (no underscores) in the
meantime.

To address the conflict, we temporarily switch to a simple comment
instead of a macro.

Related: commit f7a858bf ("tools: Rename __fallthrough to fallthrough")

Fixes: 9fd49684 ("bpftool: Support inline annotations when dumping the CFG of a program")
Reported-by: Sven Schnelle <svens@linux.ibm.com>
Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/all/yt9dttxlwal7.fsf@linux.ibm.com/
Link: https://lore.kernel.org/bpf/20230412123636.2358949-1-tmricht@linux.ibm.com/
Link: https://lore.kernel.org/bpf/20230420003333.90901-1-quentin@isovalent.com

tools/bpf/bpftool/btf_dumper.c

@@ -835,7 +835,7 @@ static void dotlabel_puts(const char *s)
 		case '|':
 		case ' ':
 			putchar('\\');
-			__fallthrough;
+			/* fallthrough */
 		default:
 			putchar(*s);
 		}