frag6.c: do not leak packet queue entry in error case
When we are checking for the maximum reassembled packet size of the
fragmentable part and run into the error case (packet too big),
we are leaking the packet queue enntry if this was a first fragment
Properly cleanup, removing the queue entry from the bucket, decrementing
counters, and freeing the memory.
MFC after: 3 weeks
Sponsored by: Netflix