targ: Handle errors from suword()

In targstart() we are already handling an error and have no go way to
signal the failure to upper layers, so ignore the return value of
suword() there.

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

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43202

(cherry picked from commit 904cd456f0f95c10fdb4c99a0118f30d23242dec)