datapath: Hash and compare only the part of sw_flow_key actually used.

datapath: Hash and compare only the part of sw_flow_key actually used.

Currently the whole flow key struct is hashed on every packet received from the
network or userspace. The whole struct is also compared byte-for-byte when
doing flow table lookups. This consumes a fair percentage of CPU time, and most
of the time part of the structure is unused (e.g. the IPv6 fields when handling
IPv4 traffic; the IPv4 fields when handling Ethernet frames).

This commit reorders the fields in the flow key struct to put the least
commonly used elements at the end and changes the hash and comparison functions
to look only at the part that contains data.

Signed-off-by: Andrew Evans <aevans@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>