ipv6: allow userspace to add IFA_F_OPTIMISTIC addresses

ipv6: allow userspace to add IFA_F_OPTIMISTIC addresses

According to RFC 4429 (section 3.1), adding new IPv6 addresses as
optimistic addresses is acceptable, as long as the implementation
follows some rules:

   * Optimistic DAD SHOULD only be used when the implementation is aware
        that the address is based on a most likely unique interface
        identifier (such as in [RFC2464]), generated randomly [RFC3041],
        or by a well-distributed hash function [RFC3972] or assigned by
        Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315].
        Optimistic DAD SHOULD NOT be used for manually entered
        addresses.

Thus, it seems reasonable to allow userspace to set the optimistic flag
when adding new addresses.

We must not let userspace set NODAD + OPTIMISTIC, since if the kernel is
not performing DAD we would never clear the optimistic flag. We must
also ignore userspace's request to add OPTIMISTIC flag to addresses that
have already completed DAD (addresses that don't have the TENTATIVE
flag, or that have the DADFAILED flag).

Then we also need to clear the OPTIMISTIC flag on permanent addresses
when DAD fails. Otherwise, IFA_F_OPTIMISTIC addresses added by userspace
can still be used after DAD has failed, because in
ipv6_chk_addr_and_flags(), IFA_F_OPTIMISTIC overrides IFA_F_TENTATIVE.

Setting IFA_F_OPTIMISTIC from userspace is conditional on
CONFIG_IPV6_OPTIMISTIC_DAD and the optimistic_dad sysctl.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>