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