MINOR: quic: refactor packet drop on reception

MINOR: quic: refactor packet drop on reception

Sometimes, a packet is dropped on reception. Several goto statements are
used, mostly to increment a proxy drop counter or drop silently the
packet. However, this labels are interleaved. Re-arrang goto labels to
simplify this process :
* drop label is used to drop a packet with counter incrementation. This
  is the default method.
* drop_silent is the next label which does the same thing but skip the
  counter incrementation. This is useful when we do not need to report
  the packet dropping operation.

This should be backported up to 2.6.