ipv6: Add icmp_echo_ignore_all support for ICMPv6

Preventing the kernel from responding to ICMP Echo Requests messages
can be useful in several ways. The sysctl parameter
'icmp_echo_ignore_all' can be used to prevent the kernel from
responding to IPv4 ICMP echo requests. For IPv6 pings, such
a sysctl kernel parameter did not exist.

Add the ability to prevent the kernel from responding to IPv6
ICMP echo requests through the use of the following sysctl
parameter : /proc/sys/net/ipv6/icmp/echo_ignore_all.
Update the documentation to reflect this change.

Signed-off-by: Virgile Jarry <virgile@acceis.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>