mptcp: Do not allocate mptcp_tcp_sock for meta

It is actually completly useless. We do not need it and it is almost
nowhere used. Except, in mptcp_established_options, when called from
tcp_current_mss on the meta-sk. But there, it is completly pointless to
call mptcp_established_options, because the time when the segment gets
sent, the subflow will be different and so the mss-info is obsolete.

So, we can get rid of meta_tp->mptcp. Simply, we do not allocate it, and
thus needed to change the parts of the code where it is freed.

This gives us a performance-increase of about 3% in request/response
stress-tests where the request/response fits in one single packet:

Before:
7621 req/s

After:
7880 req/s

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Signed-off-by: Sébastien Barré <sebastien.barre@uclouvain.be>