Closing the file after mmap will reduce the number of files associated
with the process. This will not likely help with #71698 but it doesn't
hurt to close the files and should simplify lsof output.
For #71698
Change-Id: I06a1bf91914afc7703783fe1a38d8bc5a6fb3d9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/653055
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>