openvswitch: Allow for any level of nesting in flow attributes

nlattr_set() is currently hardcoded to two levels of nesting. This change
introduces struct ovs_len_tbl to define minimal length requirements plus
next level nesting tables to traverse the key attributes to arbitrary depth.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>