tcp: tcp_mtu_probing() cleanup

Reduce one indentation level to make code more readable.
tcp_sync_mss() can be factorized.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>