IB/uverbs: Add flow_action create and destroy verbs

A verbs application may receive and transmits packets using a data
path pipeline. Sometimes, the first stage in the receive pipeline or
the last stage in the transmit pipeline involves transforming a
packet, either in order to make it easier for later stages to process
it or to prepare it for transmission over the wire. Such transformation
could be stripping/encapsulating the packet (i.e. vxlan),
decrypting/encrypting it (i.e. ipsec), altering headers, doing some
complex FPGA changes, etc.

Some hardware could do such transformations without software data path
intervention at all. The flow steering API supports steering a
packet (either to a QP or dropping it) and some simple packet
immutable actions (i.e. tagging a packet). Complex actions, that may
change the packet, could bloat the flow steering API extensively.
Sometimes the same action should be applied to several flows.
In this case, it's easier to bind several flows to the same action and
modify it than change all matching flows.

Introducing a new flow_action object that abstracts any packet
transformation (out of a standard and well defined set of actions).
This flow_action object could be tied to a flow steering rule via a
new specification.

Currently, we support esp flow_action, which encrypts or decrypts a
packet according to the given parameters. However, we present a
flexible schema that could be used to other transformation actions tied
to flow rules.

Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>