systm: Relax __result_use_check annotations

[markj]

Summary

When compiling with gcc, functions annotated this way can not have their
return values cast away, e.g., with (void)copyout(...). clang permits
it but gcc does not. Since we have a number of such casts for calls
which copy data out of the kernel, and since failing to check for errors
when copying *in* is a much larger problem, remove some of the
annotations in order to make the gcc build happy.

Reported by: Jenkins
Fixes: 8e36732e6eb5 ("systm: Annotate copyin() and related functions with __result_use_check")

[kib]

Would it make more sense to change __result_use_check to be off for gcc for now?
It would be also useful to have some symbol that turn them forcibly on/off.

[markj]

In D43418#989890, @kib wrote:

Would it make more sense to change __result_use_check to be off for gcc for now?
It would be also useful to have some symbol that turn them forcibly on/off.

I think my original change was just too aggressive. It's sometimes ok to ignore the return value of copyout(), and gcc documentation for this attribute states, "This is useful for functions where not checking the result is either a security problem or always a bug". This description applies to copyin(), but not to copyout().