sa: Check for errors from copyout()

This is in preparation for annotating copyin() and related functions
with __result_use_check.

Reviewed by: ken
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43203

(cherry picked from commit 4d3c234d8eb32f903d630b371208a0a931d3d5fe)