seg6: ignore routing header with segments left equal to 0

When there are 2 segments routing header, after an End.B6 action
for example, the second SRH will never be handled by an action, packet will
be dropped when the first SRH has segments left equal to 0.
For actions that doesn't perform decapsulation (currently: End, End.X,
End.T, End.B6, End.B6.Encaps), this patch adds the IP6_FH_F_SKIP_RH flag
in arguments for ipv6_find_hdr().

Signed-off-by: Julien Massonneau <julien.massonneau@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>