mptcp: ensure snd_una is properly initialized on connect

In the Linux kernel, the following vulnerability has been resolved:

mptcp: ensure snd_una is properly initialized on connect

This is strictly related to commit fb7a0d334894 ("mptcp: ensure snd_nxt

is properly initialized on connect"). It turns out that syzkaller can

trigger the retransmit after fallback and before processing any other

incoming packet - so that snd_una is still left uninitialized.

Address the issue explicitly initializing snd_una together with snd_nxt

and write_seq.