runtime: optimize the memory padding in p struct

Since allocation for p struct will be rounded up to the next size class,
the two relevant adjacent classes for this case are 9728 bytes and 10240 bytes.

A p is currently 10072 bytes, so it gets rounded up to 10240 bytes when we allocate one,
So the pad in p struct is unnecessary, eliminate it and add comments for
warning the false sharing.

Change-Id: Iae8b32931d1beddbfff1f58044d8401703da6407
Reviewed-on: https://go-review.googlesource.com/c/go/+/268759


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>