pkg/build/openbsd: sync before umount (#2414)
pkg/build/openbsd: sync before umount (#2414)

Reported-by: 68ee7be343cecf73080dd6921aca91aa050f2948@syzkaller.appspotmail.com
google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller