bonding: use unspecified address if no available link local address

bonding: use unspecified address if no available link local address

When ns_ip6_target was set, the ipv6_dev_get_saddr() will be called to get
available source address and send IPv6 neighbor solicit message.

If the target is global address, ipv6_dev_get_saddr() will get any
available src address. But if the target is link local address,
ipv6_dev_get_saddr() will only get available address from our interface,
i.e. the corresponding bond interface.

But before bond interface up, all the address is tentative, while
ipv6_dev_get_saddr() will ignore tentative address. This makes we can't
find available link local src address, then bond_ns_send() will not be
called and no NS message was sent. Finally bond interface will keep in
down state.

Fix this by sending NS with unspecified address if there is no available
source address.

Reported-by: LiLiang <liali@redhat.com>
Fixes: 5e1eeef69c0f ("bonding: NS target should accept link local address")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>