net: skbuff: don't zero tc members when freeing skb

Not needed, only four cases:
 - kfree_skb (or one of its aliases).
   Don't need to zero, memory will be freed.
 - kfree_skb_partial and head was stolen:  memory will be freed.
 - skb_morph:  The skb header fields (including tc ones) will be
   copied over from the 'to-be-morphed' skb right after
   skb_release_head_state returns.
 - skb_segment:  Same as before, all the skb header
   fields are copied over from the original skb right away.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>