quic: fix race condition in fast-path writes

Closing a stream caused racy access to the fast-path output buffer.
Add a mutex guarding the buffer. The mutex is uncontended except
when closing.

For #78737

Change-Id: I6e964a4564a5fb531305797f8c39eead6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/766940
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Damien Neil <dneil@google.com>