[release-branch.go1.12] internal/poll: fix deadlock in Write if len(buf) > maxRW

fd.l.Lock shouldn't be called in a loop.

Manual backport of CL 165598. It could not be cherry-picked due to conflicts.

Fixes #31211

Change-Id: Ib76e679f6a276b32fe9c1594b7e9a506017a7967
Reviewed-on: https://go-review.googlesource.com/c/go/+/170680


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>