iwlwifi: mvm: synchronize with FW after multicast commands

If userspace installs a lot of multicast groups very quickly, then
we may run out of command queue space as we send the updates in an
asynchronous fashion (due to locking concerns), and the CPU can
create them faster than the firmware can process them. This is true
even when mac80211 has a work struct that gets scheduled.

Fix this by synchronizing with the firmware after sending all those
commands - outside of the iteration we can send a synchronous echo
command that just has the effect of the CPU waiting for the prior
asynchronous commands to finish. This also will cause fewer of the
commands to be sent to the firmware overall, because the work will
only run once when rescheduled multiple times while it's running.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=213649


Suggested-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reported-by: Maximilian Ernestus <maximilian@ernestus.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20211204083238.51aea5b79ea4.I88a44798efda16e9fe480fb3e94224931d311b29@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>