Calling fs.Sub with the result of fs.Sub multiple times creates a deep
call stack for Open and other methods. Enhance the fs.FS returned by
fs.Sub to implement fs.SubFS and reduce the call stack.
Fixes #45349
Change-Id: I10e10501e030176e10e2ae5ad260212e5c784bed
Reviewed-on: https://go-review.googlesource.com/c/go/+/306769
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>