Adds the new procfs knobs:

    /proc/sys/net/ipv4/conf/*/igmpv2_unsolicited_report_interval
    /proc/sys/net/ipv4/conf/*/igmpv3_unsolicited_report_interval

Which will allow userspace configuration of the IGMP unsolicited report
interval (see below) in milliseconds.  The defaults are 10000ms for IGMPv2
and 1000ms for IGMPv3 in accordance with RFC2236 and RFC3376.

Background:

If an IGMP join packet is lost you will not receive data sent to the
multicast group so if no data arrives from that multicast group in a
period of time after the IGMP join a second IGMP join will be sent.  The
delay between joins is the "IGMP Unsolicited Report Interval".

Prior to this patch this value was hard coded in the kernel to 10s for
IGMPv2 and 1s for IGMPv3.  10s is unsuitable for some use-cases, such as
IPTV as it can cause channel change to be slow in the presence of packet
loss.

This patch allows the value to be overridden from userspace for both
IGMPv2 and IGMPv3 such that it can be tuned accoding to the network.

Tested with Wireshark and a simple program to join a (non-existent)
multicast group.  The distribution of timings for the second join differ
based upon setting the procfs knobs.

igmpvX_unsolicited_report_interval is intended to follow the pattern
established by force_igmp_version, and while a procfs entry has been added
a corresponding sysctl knob has not as it is my understanding that sysctl
is deprecated[1].

[1]: http://lwn.net/Articles/247243/Signed-off-by: William Manley <william.manley@youview.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: David S. Miller <davem@davemloft.net>