cfm: No longer keep track of bad CCMs.

According to the 802.1ag specification, reception of a CCM from an
unexpected source should trigger a fault. This patch causes the CFM
module to simply warn instead.  There are several reasons for this
change outlined below.

  - Faults can cause controllers to make potentially expensive
    changes to the network topology.
  - Faults can be maliciously triggered by crafting invalid CCMs.
  - With this patch, cfm->fault and rmp->fault are only updated in
    cfm_run() making the code easier to debug and reason about.