Not properly reporting this error can result in values "returned" from
`sbytes` being misinterpreted by the caller.
When reporting `sbytes`, be sure to not squash existing errors outside of
EAGAIN, EBUSY, and EINTR. These errors are non-fatal, so they should only be
percolated up on failure if EFAULT isn't reported, as EFAULT affects the
value of `sbytes`.
MFC after: 2 weeks
Sponsored by: Netflix, Inc