ipv6: Remove IPV6_ADDR_RESERVED

RFC 4291 section 2.4 states that all uncategorized addresses
should be considered as Global Unicast.

This will remove IPV6_ADDR_RESERVED completely
and return IPV6_ADDR_UNICAST in ipv6_addr_type() instead.

Signed-off-by: Ulrich Weber <uweber@astaro.com>
Signed-off-by: David S. Miller <davem@davemloft.net>