datapath: Convert odp_flow_key to use Netlink attributes instead.

One of the goals for Open vSwitch is to decouple kernel and userspace
software, so that either one can be upgraded or rolled back independent of
the other.  To do this in full generality, it must be possible to change
the kernel's idea of the flow key separately from the userspace version.
In turn, that means that flow keys must become variable-length.  This
commit makes that change using Netlink attribute sequences.

This commit does not actually make userspace flexible enough to handle
changes in the kernel flow key structure, because userspace doesn't yet
have enough information to do that intelligently.  Upcoming commits will
fix that.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>