crypto/rand: fix race on r.used
crypto/rand: fix race on r.used

This race is benign, but it still trips up the race detector, so turn
this into an atomic read.

Fixes #52739.

Change-Id: Ib53362286b456513c8c69d6d2d73c6c90ec095f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/404475
Auto-Submit: Jason Donenfeld <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Jason Donenfeld <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>

nawawi/go,nawawi/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,golang/go