This change reduces the overhead of calling routing information per IPv6
link-local datagram read by caching IPv6 addressing scope zone
information.

Fixes #15237.

name                    old time/op    new time/op    delta
UDP6LinkLocalUnicast-8    64.9µs ± 0%    18.6µs ± 0%  -71.30%

name                    old alloc/op   new alloc/op   delta
UDP6LinkLocalUnicast-8    11.2kB ± 0%     0.2kB ± 0%  -98.42%

name                    old allocs/op  new allocs/op  delta
UDP6LinkLocalUnicast-8       101 ± 0%         3 ± 0%  -97.03%

Change-Id: I5ae2ef5058df1028bbb7f4ab32b13edfb330c3a7
Reviewed-on: https://go-review.googlesource.com/21952Reviewed-by: Ian Lance Taylor <iant@golang.org>