ATM: mpc, fix use after free

Stanse found that mpc_push frees skb and then it dereferences it. It
is a typo, new_skb should be dereferenced there.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>