tcp: enforce tcp_min_snd_mss in tcp_mtu_probing()

tcp: enforce tcp_min_snd_mss in tcp_mtu_probing()

If mtu probing is enabled tcp_mtu_probing() could very well end up
with a too small MSS.

Use the new sysctl tcp_min_snd_mss to make sure MSS search
is performed in an acceptable range.

CVE-2019-11479 -- tcp mss hardcoded to 48

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Cc: Jonathan Looney <jtl@netflix.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Tyler Hicks <tyhicks@canonical.com>
Cc: Bruce Curtis <brucec@netflix.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name tcp-enforce-tcp_min_snd_mss-in-tcp_mtu_probing.patch