ipv6: fix net.ipv6.conf.all interface DAD handlers

Currently, writing into
net.ipv6.conf.all.{accept_dad,use_optimistic,optimistic_dad} has no effect.
Fix handling of these flags by:

- using the maximum of global and per-interface values for the
  accept_dad flag. That is, if at least one of the two values is
  non-zero, enable DAD on the interface. If at least one value is
  set to 2, enable DAD and disable IPv6 operation on the interface if
  MAC-based link-local address was found

- using the logical OR of global and per-interface values for the
  optimistic_dad flag. If at least one of them is set to one, optimistic
  duplicate address detection (RFC 4429) is enabled on the interface

- using the logical OR of global and per-interface values for the
  use_optimistic flag. If at least one of them is set to one,
  optimistic addresses won't be marked as deprecated during source address
  selection on the interface.

While at it, as we're modifying the prototype for ipv6_use_optimistic_addr(),
drop inline, and let the compiler decide.

Fixes: 7fd2561e

 ("net: ipv6: Add a sysctl to make optimistic addresses useful candidates")
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>