bpftool: Use "fallthrough;" keyword instead of comments

After using "__fallthrough;" in a switch/case block in bpftool's
btf_dumper.c [0], and then turning it into a comment [1] to prevent a
merge conflict in linux-next when the keyword was changed into just
"fallthrough;" [2], we can now drop the comment and use the new keyword,
no underscores.

Also update the other occurrence of "/* fallthrough */" in bpftool.

[0] commit 9fd49684 ("bpftool: Support inline annotations when dumping the CFG of a program")
[1] commit 4b7ef71a ("bpftool: Replace "__fallthrough" by a comment to address merge conflict")
[2] commit f7a858bf ("tools: Rename __fallthrough to fallthrough")
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230712152322.81758-1-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);
 		}

tools/bpf/bpftool/feature.c

@@ -757,7 +757,7 @@ probe_helpers_for_progtype(enum bpf_prog_type prog_type,
 		case BPF_FUNC_probe_write_user:
 			if (!full_mode)
 				continue;
-			/* fallthrough */
+			fallthrough;
 		default:
 			probe_res |= probe_helper_for_progtype(prog_type, supported_type,
 						  define_prefix, id, prog_type_str,