This is an RFC patch to add better IPv6 failover support for bonding 
devices, especially when in active-backup mode, as reported by Alex 
Sidorenko.

What this patch does:

- Creates a new Kconfig option in the IPv6 Networking section to
   compile-in the support in the bonding driver.  This also forces
   IPV6=y since that's required to link everything.

- Creates a new file, net/drivers/bonding/bond_ipv6.c, for the
   IPv6-specific routines.

- Adds a new master_ipv6 address member to the bonding struct to
   hold a copy of the primary IPv6 address on the bond.

- Adds a new tunable, num_grat_ns, to limit the number of gratuitous
   Neighbor Solicitations that are sent on a failover event.  Default
   is 1.

On failover, this new code will generate two packets:

- An MLD report for the bond, on the current active slave.

- An IPv6 "gratuitous" Neighbor Solicitation, which helps the switch
   learn that the address has moved to the new slave.

Testing has shown that sending just the NS results in pretty good 
behavior when in active-back mode, I saw no lost ping packets for 
example.  Sending just the MLD packet didn't seem to have the same 
effect.  Sending both seems like the right thing to do.

Comments welcome.

-Brian

Signed-off-by: Brian Haley <brian.haley@hp.com>
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