ethdev: add capability to keep flow rules on restart

ethdev: add capability to keep flow rules on restart

Previously, it was not specified what happens to the flow rules
when the device is stopped, possibly reconfigured, then started.
If flow rules were kept, it could be convenient for application
developers, because they wouldn't need to save and restore them.
However, due to the number of flows and possible creation rate it is
impractical to save all flow rules in DPDK layer. This means that flow
rules persistence really depends on whether PMD and HW can implement it
efficiently. It can also be limited by the rule item and action types,
and its attributes transfer bit (a combination of an item/action type
and a value of the transfer bit is called a rule feature).

Add a device capability bit for PMDs that can keep at least some
of the flow rules across restart. Without this capability behavior
is still unspecified and it is declared that the application must
flush the rules before stopping the device.
Allow the application to test for persistence of rules using
a particular feature by attempting to create a flow rule
using that feature when the device is stopped
and checking for the specific error.
This is logical because if the PMD can to create the flow rule
when the device is not started and use it after the start happens,
it is natural that it can move its internal flow rule object
to the same state when the device is stopped and restore the state
when the device is started.

Rule persistence across a reconfigurations is not required,
because tracking all the rules and configuration-dependent resources
they use may be infeasible. In case a PMD cannot keep the rules
across reconfiguration, it is allowed just to report an error.
Application must then flush the rules before attempting it.

Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>