tcp: reinitialize MTU probing when setting MSS in a TCP repair

MTU probing initialization occurred only at connect() and at SYN or
SYN-ACK reception, but the former sets MSS to either the default or the
user set value (through TCP_MAXSEG sockopt) and the latter never happens
with repaired sockets.

The result was that, with MTU probing enabled and unless TCP_MAXSEG
sockopt was used before connect(), probing would be stuck at
tcp_base_mss value until tcp_probe_interval seconds have passed.

Signed-off-by: Douglas Caetano dos Santos <douglascs@taghos.com.br>
Signed-off-by: David S. Miller <davem@davemloft.net>