[op] Make gofer client handle return partial write length when err is nil.

If there was a partial write (when not using the host FD) which did not generate
an error, we were incorrectly returning the number of bytes attempted to write
instead of the number of bytes actually written.

PiperOrigin-RevId: 363058989

pkg/sentry/fsimpl/gofer/handle.go

@@ -124,8 +124,9 @@ func (h *handle) writeFromBlocksAt(ctx context.Context, srcs safemem.BlockSeq, o
 		return 0, cperr
 	}
 	n, err := h.file.writeAt(ctx, buf[:cp], offset)
+	// err takes precedence over cperr.
 	if err != nil {
 		return uint64(n), err
 	}
-	return cp, cperr
+	return uint64(n), cperr
 }