Fix: mptcp_current_mss to return the minimum

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Cleanup mptcp_select_size

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Set data_len manually when receiving infinite mapping

As data_len == 0 to signal the infinite mapping, we have to set it
manually to skb->len in order to allow correct processing afterwards.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>