hash/crc32: fix race between lazy Castagnoli init and Update/Write

The switch on tab is checking tab == castagnoliTable,
but castagnoliTable can change value during a concurrent
call to MakeTable.

Fixes #41911.

Change-Id: I6124dcdbf33e17fe302baa3e1aa03202dec61b4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/261639


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>