ipv6 addrconf: add IFA_F_NOPREFIXROUTE flag to suppress creation of IP6 routes

When adding/modifying an IPv6 address, the userspace application needs
a way to suppress adding a prefix route. This is for example relevant
together with IFA_F_MANAGERTEMPADDR, where userspace creates autoconf
generated addresses, but depending on on-link, no route for the
prefix should be added.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>