Truncate changes the size of the file. It does not change the I/O offset.

Fixes #29757

Change-Id: I1aa9223a86d6a8ce3c0efc3ac1d7d7647b77f589
Reviewed-on: https://go-review.googlesource.com/c/158117Reviewed-by: Bryan C. Mills <bcmills@google.com>