openvswitch: Sample action without side effects

The sample action is rather generic, allowing arbitrary actions to be
executed based on a probability. However its use, within the Open
vSwitch
code-base is limited: only a single user-space action is ever nested.

A consequence of the current implementation of sample actions is that
depending on weather the sample action executed (due to its probability)
any side-effects of nested actions may or may not be present before
executing subsequent actions.  This has the potential to complicate
verification of valid actions by the (kernel) datapath. And indeed
adding support for push and pop MPLS actions inside sample actions
is one case where such case.

In order to allow all supported actions to be continue to be nested
inside sample actions without the potential need for complex
verification code this patch changes the implementation of the sample
action in the kernel datapath so that sample actions are more like
a function call and any side effects of nested actions are not
present when executing subsequent actions.

With the above in mind the motivation for this change is twofold:

* To contain side-effects the sample action in the hope of making it
  easier to deal with in the future and;
* To avoid some rather complex verification code introduced in the MPLS
  datapath patch.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>