oce(4): Don't directly access usespace

Replace direct stores to userspace addresses (never safe and broken on
modern CPUs) with a copyout. Use a static assert on the size to ensure
we don't overflow the field.

Reviewed by: markj, jhb
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D40519

(cherry picked from commit 758927a982db0ae3ebb02c05aec8a53bcc0c20cb)