net.server_epoll: Prevent stack overflow of opportunistic writes

net.http.files serving a big enough file on a fast enough connection
with opportunistic_writes enabled could trigger a stack overflow through
repeatedly serving more data that immediately gets sent, draining the
buffer and triggering more data to be sent. This also blocked the server
on a single task until completion or an error.

This change prevents nested opportunistic writes, which should prevent
the stack overflow, at the cost of reduced download speed, but this is
unlikely to be noticeable outside of Gbit networks. Speed at the cost of
blocking other processing is not worth it, especially with the risk of
stack overflow.

net/server_epoll.lua

@@ -499,8 +499,10 @@ function interface:write(data)
 		self.writebuffer = { data };
 	end
 	if not self._write_lock then
-		if cfg.opportunistic_writes then
+		if cfg.opportunistic_writes and not self._opportunistic_write then
+			self._opportunistic_write = true;
 			self:onwritable();
+			self._opportunistic_write = nil;
 			return #data;
 		end
 		self:setwritetimeout();