openvswitch: Validate IPv6 flow key and mask values.

Reject flow label key and mask values with invalid bits set.
Introduced by commit 3fdbd1ce ("openvswitch: add ipv6 'set'
action").

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>