mptcp_skb_entail(): add unlikely

[matttbe]

Some conditions should rarely happen in mptcp_skb_entail(), e.g. mptcp_is_data_fin(), mpcb->send_mptcpv1_mpcapable, etc.

Better to tell that to the compiler by adding likely() / unlikely() macros not to take the more costly branch most of the time in this important function.