systm: Annotate copyin() and related functions with __result_use_check

systm: Annotate copyin() and related functions with __result_use_check

Now that all in-tree callers check for errors (or cast them away), we
can ask the compiler to check that new code does the same.  This was
prompted by SA-23:18.nfsclient, which was caused by missing error
handling.  This change is a weak mitigation since code can easily fail
to propagate error handling to the right place, but it's better than
nothing.

Reviewed by:	kib, jhb
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D43223