scripts/kernel-doc crashes (undeclared variable)

[AnthonyKirby]

/usr/src/linux-headers-5.4.0-187/scripts/kernel-doc has a syntax/compilation error, which makes the script fail.

To reproduce:
- install linux-headers-5.4.0-187
- run /usr/src/linux-headers-5.4.0-187/scripts/kernel-doc

What should happen:
- should print usage doc

What happens:
- fails with error
```
Global symbol "$args" requires explicit package name (did you forget to declare "my $args"?) at /usr/src/linux-headers-5.4.0-187/scripts/kernel-doc line 1089.
Global symbol "$args" requires explicit package name (did you forget to declare "my $args"?) at /usr/src/linux-headers-5.4.0-187/scripts/kernel-doc line 1089.
Execution of /usr/src/linux-headers-5.4.0-187/scripts/kernel-doc aborted due to compilation errors.
```

Details about the syntax error:
- this is a regression since package linux-headers-5.4.0-182: script /usr/src/linux-headers-5.4.0-182/scripts/kernel-doc works OK & prints usage doc
- the error message is a clear representation of the problem : in function "dump_struct($$)", "args" has not been declared when used at line 1089

What was I doing that made me notice this?:
- building a DKMS module; the module failed to build on this kernel version.

Why do I care?
- I had to patch the script in linux-headers in order to build a DKMS module. It's clearly a bug in the kernel-doc script (being an undeclared variable), not a bug in the DKMS module. I'm keen not to have to patch linux-headers-<$future-version> in order to support the deployment of the DKMS module in my organisation.

How do I think this happened?
- looking at current kernel source:
- - "my $args" declared at https://github.com/torvalds/linux/blob/master/scripts/kernel-doc#L1160
- - "$members =~ s/(?:__)?DECLARE_FLEX_ARRAY\s*\($args,\s*$args\)/$1 $2\[\]/gos;" referenced at https://github.com/torvalds/linux/blob/master/scripts/kernel-doc#L1172
- I suspect that the usage line was cherry-picked, but the refactor to use $args hadn't been merged
- i.e. something like https://github.com/torvalds/linux/commit/3080ea5553cc909b000d1f1d964a9041962f2c5b was cherry-picked, but relied on a refactor in https://github.com/torvalds/linux/commit/e86bdb24375a810ea7993d64ed406a803db71225

How to fix?
- the simple fix would be remove lines 1088 & 1089, i.e. revert to linux-headers-5.4.0-182
- add a check that this script is valid, run for future builds (given that it's not otherwise covered)

[AnthonyKirby (a3thony)]

Also affects kernel 5.4.0-189

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.