Polling on a tcp socket should not return if the sendbuffer is full.
Polling on a tcp socket should not return if the sendbuffer is full.

This fixes an off-by-one error where polling on a tcp socket could
return POLLOUT when the sendbuffer is actually full, and a write would
return EWOULDBOCK.

PiperOrigin-RevId: 174239388

fuchsia-mirror/third_party-netstack,google/netstack