ofproto-dpif-upcall: Don't delete modified ukeys.

ofproto-dpif-upcall: Don't delete modified ukeys.

If revalidation returns the result UKEY_DELETE, then both the ukey and
its corresponding flow should be deleted. However, if revalidation
returns UKEY_MODIFY, the ukey itself should be modified in-place and
should not be deleted.

Fix this by only applying the ukey deletion to ukeys whose datapath
operations delete a flow.

This may fix statistics accounting issues in rare cases involving
OpenFlow rule modification where actions are updated but flows remain
the same.

Found by inspection.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>