Do not use pkfree_skb events for synchronization

Don't rely on events indicating when sk_buff structures are freed. After a
receive, we wait for another event indicating that this receive was for TCP
data. In the case where the data was not TCP, instead of keeping information
about the receive, we used to discard it when the skb was freed.  It turns out
that it faster (and simpler) not to look at pkfree_skb events and keep the
information around anyways. Since sk_buff's are allocated in a pool, the
information will get overwritten and the size of pendingRecv will not grow
infinitely.

Signed-off-by: Benjamin Poirier <benjamin.poirier@polymtl.ca>