net: use internal/bytealg.CountString

net: use internal/bytealg.CountString

On platforms that provide a native implementation this might be slightly
faster. On other platforms it is equivalent to the count func.

Change-Id: If46cc65598993e64084cc98533cb8c1e9679a6fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/522136
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>