net: use internal/bytealg insetad of linkname tricks

We are currently using go:linkname for some algorithms from
strings/bytes packages, to avoid importing strings/bytes.
But strings/bytes are just wrappers around internal/bytealg, so
we should use internal/bytealg directly.

Change-Id: I2836f779b88bf8876d5fa725043a6042bdda0390
Reviewed-on: https://go-review.googlesource.com/130515


Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>