net: fib_rules: Add flow label selector attributes

net: fib_rules: Add flow label selector attributes

Add new FIB rule attributes which will allow user space to match on the
IPv6 flow label with a mask. Temporarily set the type of the attributes
to 'NLA_REJECT' while support is being added in the IPv6 code.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>