conn: fix read buffer size for batches with echo messages

[rootkiller6788]

What

enlargeReadBuffer in conn.go had its echo-message detection inverted. The comment says the default (large) read buffer should be used when a batch contains messages with the Echo flag, but the code applied it when no message had the flag:

if msg.Header.Flags&netlink.Echo == 0 {
    bufferSize = cc.getDefaultEchoReadBuffer()
    break
}

Why it matters

Rule creation messages (AddRule/InsertRule) set netlink.Echo so the kernel echoes back each created rule. For those batches the code sized the socket receive buffer at only len(messages) * 1024, which can be too small for large batches and produce netlink receive: recvmsg: no buffer space available (see #350). Batches without echo messages, on the other hand, were given the multi-megabyte default buffer unnecessarily.

Fix

• Invert the condition to Flags&netlink.Echo != 0.
• Extract the buffer-size computation into a readBufferSize() method so it can be unit tested without a kernel.
• Add TestReadBufferSize covering echo-less, echo, and large-echo batches.

Verified with the full unit test suite (system tests excluded); the new test fails on the pre-fix code and passes with the fix.