linux: Check for copyout errors in linux_fixup()

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

Reviewed by: olce, dchagin
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43104

(cherry picked from commit 3fde275167ce1f1455a03586d29840546d06d97a)