syscall: avoid serializing forks on ForkLock

syscall: avoid serializing forks on ForkLock

Fixes #23558
Fixes #54162

Change-Id: I3cf6efe466080cdb17e171218e9385ccb272c301
Reviewed-on: https://go-review.googlesource.com/c/go/+/421441
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>