log/slog: fix issue with concurrent writes

log/slog: fix issue with concurrent writes

This causes instances commonHandler created by withAttrs or withGroup to
share a mutex with their parent preventing concurrent writes to their
shared writer.

Fixes #61321

Change-Id: Ieec225e88ad51c84b41bad6c409fac48c90320b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/509196
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>