net: use mid-stack inlining with ReadFromUDP to avoid an allocation

This commit rewrites ReadFromUDP to be mid-stack inlined
and pass a UDPAddr for lower layers to fill in.

This lets performance-sensitive clients avoid an allocation.
It requires some care on their part to prevent the UDPAddr
from escaping, but it is now possible.
The UDPAddr trivially does not escape in the benchmark,
as it is immediately discarded.

name                  old time/op    new time/op    delta
WriteToReadFromUDP-8    17.2µs ± 6%    17.1µs ± 5%     ~     (p=0.387 n=9+9)

name                  old alloc/op   new alloc/op   delta
WriteToReadFromUDP-8      112B ± 0%       64B ± 0%  -42.86%  (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
WriteToReadFromUDP-8      3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.000 n=10+10)

Updates #43451

Co-authored-by: Filippo Valsorda <filippo@golang.org>
Change-Id: I1f9d2ab66bd7e4eff07fe39000cfa0b45717bd13
Reviewed-on: https://go-review.googlesource.com/c/go/+/291509


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Filippo Valsorda <filippo@golang.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>