net/multicast: should not send source list records when have filter mode change

net/multicast: should not send source list records when have filter mode change

Based on RFC3376 5.1 and RFC3810 6.1

   If the per-interface listening change that triggers the new report is
   a filter mode change, then the next [Robustness Variable] State
   Change Reports will include a Filter Mode Change Record.  This
   applies even if any number of source list changes occur in that
   period.

   Old State         New State         State Change Record Sent
   ---------         ---------         ------------------------
   INCLUDE (A)       EXCLUDE (B)       TO_EX (B)
   EXCLUDE (A)       INCLUDE (B)       TO_IN (B)

So we should not send source-list change if there is a filter-mode change.

Here are two scenarios:
1. Group deleted and filter mode is EXCLUDE, which means we need send a
   TO_IN { }.
2. Not group deleted, but has pcm->crcount, which means we need send a
   normal filter-mode-change.

At the same time, if the type is ALLOW or BLOCK, and have psf->sf_crcount,
we stop add records and decrease sf_crcount directly

Reference: https://www.ietf.org/mail-archive/web/magma/current/msg01274.html

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>