ipv6: add inet6_fill_args

inet6_fill_if{addr,mcaddr, acaddr}() already took 6 arguments which
meant the 7th argument would need to be pushed onto the stack on x86.
Add a new struct inet6_fill_args which holds common information passed
to inet6_fill_if{addr,mcaddr, acaddr}() and shortens the functions to
three pointer arguments.

Signed-off-by: Christian Brauner <christian@brauner.io>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>