ofproto-dpif: Ignore non-packet field masks during flow revalidation

ofproto-dpif: Ignore non-packet field masks during flow revalidation

Commit bcd2633a5be(ofproto-dpif: Store relevant fields for wildcarding
in facet) implements flow revalidation by comparing the newly looked
up flow mask with that of the existing facet.

The non-packet fields, such as register masks, are always cleared
by xlate_actions in the masks stored within facets, but they are not
cleared in the newly looked up flow masks, causing otherwise valid
flows to be declared as invalid flows and be removed from the datapath.

This patch provides a fix. I was able to verify the fix on a system set
up by Ying Chen where the bug can be reproduced.

Bug #21680
Reported by: Ying Chen <yingchen@vmware.com>

Acked-by: Justin Pettit <jpettit@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>