dpif-netdev: Move hash function out of the recirc action, into its own action

Currently recirculation action can optionally compute hash. This patch
adds a hash action that is independent of the recirc action, which
no longer computes hash.  For megaflow bond with recirc, the output
to a bond port action will look like:

    hash(hash_l4(0)), recirc(<recirc_id>)

Obviously, when a recirculation application that does not depend on
hash value can just use the recirc action alone.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Reviewed-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Pravin B Shelar <pshelar@nicira.com