ses: Add error checking for copyout() calls

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

Reviewed by: mav, imp, asomers
Tested by: asomers
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43130

(cherry picked from commit 771501e96ffd7e314c338300a351ad76d28a2f69)