datapath: Fix MPLS action validation.

Linux stack do not allow GSO for packet with multiple
encapsulations.  Therefore there was check in MPLS action
validation to detect such case, But it is better to add
such check at run time to detect such cases.
Removing this check also fixes bug in action copy to no skip
multiple set actions.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Reported-by: Srinivas Neginhal <sneginha@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Bug #1367702