openvswitch: fix mis-ordered comment lines for ovs_skb_cb

I was trying to wrap my head around meaning of mru, and realised
that the second line of the comment defining it had somehow
ended up after the line defining cutlen, leading to much confusion.

Reorder the lines to make sense.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: David S. Miller <davem@davemloft.net>