[IPv6] rules: Use RT6_LOOKUP_F_HAS_SADDR and fix source based selectors

Fixes rt6_lookup() to provide the source address in the flow
and sets RT6_LOOKUP_F_HAS_SADDR whenever it is present in
the flow.

Avoids unnecessary prefix comparisons by checking for a prefix
length first.

Fixes the rule logic to not match packets if a source selector
has been specified but no source address is available.

Thanks to Kim Nordlund <kim.nordlund@nokia.com> for working
on this patch with me.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Ville Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>