ofproto-dpif: Use dp_hash as default selection method

ofproto-dpif: Use dp_hash as default selection method

The dp_hash selection method for select groups overcomes the scalability
problems of the current default selection method which, due to L2-L4
hashing during xlation and un-wildcarding of the hashed fields,
basically requires an upcall to the slow path to load-balance every
L4 connection. The consequence are an explosion of datapath flows
(megaflows degenerate to miniflows) and a limitation of connection
setup rate OVS can handle.

This commit changes the default selection method to dp_hash, provided the
bucket configuration is such that the dp_hash method can accurately
represent the bucket weights with up to 64 hash values. Otherwise we
stick to original default hash method.

We use the new dp_hash algorithm OVS_HASH_L4_SYMMETRIC to maintain the
symmetry property of the old default hash method.

A controller can explicitly request the old default hash selection method
by specifying selection method "hash" with an empty list of fields in the
Group properties of the OpenFlow 1.5 Group Mod message.

Update the documentation about selection method in the ovs-ovctl man page.

Revise and complete the ofproto-dpif unit tests cases for select groups.

Signed-off-by: Jan Scheurich <jan.scheurich@ericsson.com>
Signed-off-by: Nitin Katiyar <nitin.katiyar@ericsson.com>
Co-authored-by: Nitin Katiyar <nitin.katiyar@ericsson.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>