click/todevice: fix excessive skb_copy_expand of tailroom in queue_packet

This code was doing something weird, expanding tailroom to be tailroom plus
need_tail, instead of just need_tail.  This was presumably not the intent,
since tailroom is only expanded if it is less than need_tail to begin with.
click/todevice: memset tailroom after expand

After tailroom is expanded, it is full of existing data.  This is an
information leak.  I've seen pointers and (potentially-sensitive) strings
appear in this padding on the wire, which could be up to 18 bytes.
Merge pull request #102 from pallas/todevice-nits

ToDevice nits