internal/poll: fix deadlock in Write if len(buf) > maxRW
internal/poll: fix deadlock in Write if len(buf) > maxRW

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

Change-Id: I3afbc184aa06a60175c9a39319985b5810ecb144
Reviewed-on: https://go-review.googlesource.com/c/go/+/165598
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,josharian/go,akutz/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,akutz/go,josharian/go,cloudflare/go,nawawi/go,cloudflare/go,josharian/go,josharian/go,golang/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,nawawi/go,nawawi/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,josharian/go,akutz/go,golang/go,golang/go,CAFxX/go,golang/go,josharian/go,golang/go