Fix bugs with reading tcp flags and sequence

[grcevski]

While working on an experimental feature to propagate context with TCP packets, I noticed two things:

• Our socket filter code was reading a partial sequence number, I had used u16 instead of u32 for the tcp->seq. We make use this for black-box propagation.
• We don't use the TCP flags yet, but I was reading them both in the network o11y code and app o11y code after we had adjusted the current read offset after we find the total header length.