ipvs: SH fallback and L4 hashing

By default the SH scheduler rejects connections that are hashed onto a
realserver of weight 0.  This patch adds a flag to make SH choose a
different realserver in this case, instead of rejecting the connection.

The patch also adds a flag to make SH include the source port (TCP, UDP,
SCTP) in the hash as well as the source address.  This basically allows
for deterministic round-robin load balancing (i.e., where any director
in a cluster of directors with identical config will send the same
packet the same way).

The flags are service flags (IP_VS_SVC_F_SCHED*) so that these options
can be set per service.  They are set using a new option to ipvsadm.

Signed-off-by: Alexander Frolkin <avf@eldamar.org.uk>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>