systm: Relax __result_use_check annotations

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 more severe bug, remove some of the
annotations in order to make the gcc build happy.

Reviewed by: kib
Reported by: Jenkins
Fixes: 8e36732e6eb5 ("systm: Annotate copyin() and related functions with __result_use_check")
Differential Revision: https://reviews.freebsd.org/D43418