Taking a read lock in SetBlocking could cause SetBlocking to block
waiting for a Read in another goroutine to complete. Since SetBlocking
is called by os.(*File).Fd, that could lead to deadlock if the
goroutine calling Fd is going to use it to unblock the Read.
Use an atomic store instead.

Updates #24481

Change-Id: I79413328e06ddf28b6d5b8af7a0e29d5b4e1e6ff
Reviewed-on: https://go-review.googlesource.com/123176
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>