umtx: Check for errors from suword32()

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

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D43144

(cherry picked from commit 3379d9b5de4c4876a317d25ca008e66b1111b701)