net: use bytes.Equal instead of bytesEqual

net: use bytes.Equal instead of bytesEqual

bytes.Equal is written in assembly and is slightly faster than the
current Go bytesEqual from the net package.

benchcmp:
benchmark                 old ns/op     new ns/op     delta
BenchmarkIPCompare4-8     7.74          7.01          -9.43%
BenchmarkIPCompare6-8     8.47          6.86          -19.01%

Change-Id: I2a7ad35867489b46f0943aef5776a2fe1b46e2df
Reviewed-on: https://go-review.googlesource.com/36850
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>