os: reuse buffer pool more aggressively in readdir

os: reuse buffer pool more aggressively in readdir

We can reuse the buffer pool more aggressively when reading a directory
by returning the buffer to the pool as soon as we get to the end of the
directory, rather than waiting until the the os.File is closed.

This yields a significant memory usage reduction when traversing
nested directories recursively via os.File#ReadDir (and friends),
as the file pointers tends to be closed only after the entire
traversal is done. For example, this pattern is used in os.RemoveAll.
These are the improvements observed in BenchmarkRemoveAll:

goos: linux
goarch: amd64
pkg: os
cpu: AMD EPYC 7763 64-Core Processor
            │   old.txt   │            new.txt            │
            │   sec/op    │   sec/op     vs base          │
RemoveAll-4   3.847m ± 2%   3.823m ± 1%  ~ (p=0.143 n=10)

            │   old.txt    │               new.txt                │
            │     B/op     │     B/op      vs base                │
RemoveAll-4   39.77Ki ± 2%   17.63Ki ± 1%  -55.68% (p=0.000 n=10)

            │  old.txt   │              new.txt              │
            │ allocs/op  │ allocs/op   vs base               │
RemoveAll-4   510.0 ± 0%   503.0 ± 0%  -1.37% (p=0.000 n=10)

Change-Id: I70e1037378a02f1d670ccb7b275ee55f0caa6d0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/573358
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Emmanuel Odeke <emmanuel@orijtech.com>