bonding: accept unsolicited NA message

The unsolicited NA message with all-nodes multicast dest address should
be valid, as this also means the link could reach the target.

Also rename bond_validate_ns() to bond_validate_na().

Reported-by: LiLiang <liali@redhat.com>
Fixes: 5e1eeef6

 ("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>