bpf2go: improve duplicate type name error message

[wucm667]

When --type T is used for a type that is already exported via a
variable or map k/v, bpf2go reports a confusing error about the
type name being used multiple times.

Update the error message to guide users toward the fix:
remove the redundant --type flag.

Fixes #1638

[ti-mo]

@wucm667 Thanks for the patch, but I don't really agree that the behaviour should be changed. Maybe the error message is a little obtuse at the moment, that could likely be improved.

If there's already a variable or map k/v of type T, specifying it additionally using --type T will effectively make that flag dead code. It's also not ideal to couple the contents of go:generate statements to type names contained in C files. Including more than a handful of types using CLI flags sounds really cumbersome, especially when the list of types changes frequently.

For example, in Cilium we use this for exporting enums to Go using a bpf2go-like tool:

#define EXPORT_TYPE(type) __expand_type(type, __COUNTER__)
#define __expand_type(type, n) ___expand_type(type, n)
#define ___expand_type(type, n) type __dpexp_ ## n

...
enum identity {
       UNKNOWN_ID = 0,
       HOST_ID = 1,
       WORLD_ID = 2,
};
EXPORT_TYPE(enum identity);

I'd probably just update the error string in sortTypes to read something like "type name %q is used multiple times (remove its --type flag?)".

[wucm667]

@ti-mo Good point — makes sense. Updated to just improve the error message as you suggested. The --type flag is indeed dead code in this case, so guiding the user to remove it is the right approach.