BUG/MINOR: tcp: fix silent-drop workaround for IPv6

BUG/MINOR: tcp: fix silent-drop workaround for IPv6

As reported in github issue #1203 the TTL-based workaround that is used
when permissions are insufficient for the TCP_REPAIR trick does not work
for IPv6 because we're using only SOL_IP with IP_TTL. In IPv6 we have to
use SOL_IPV6 and IPV6_UNICAST_HOPS. Let's pick the right one based on the
source address's family.

This may be backported to all versions.

(cherry picked from commit ab79ee8b117dbb2c2872747e8119492e70506392)
Signed-off-by: Willy Tarreau <w@1wt.eu>