batman-adv: mcast: fix multicast tt/tvlv worker locking

commit a3c7cd0c

 upstream.

Syzbot has reported some issues with the locking assumptions made for
the multicast tt/tvlv worker: It was able to trigger the WARN_ON() in
batadv_mcast_mla_tt_retract() and batadv_mcast_mla_tt_add().
While hard/not reproduceable for us so far it seems that the
delayed_work_pending() we use might not be quite safe from reordering.

Therefore this patch adds an explicit, new spinlock to protect the
update of the mla_list and flags in bat_priv and then removes the
WARN_ON(delayed_work_pending()).

Reported-by:  <syzbot+83f2d54ec6b7e417e13f@syzkaller.appspotmail.com>
Reported-by:  <syzbot+050927a651272b145a5d@syzkaller.appspotmail.com>
Reported-by:  <syzbot+979ffc89b87309b1b94b@syzkaller.appspotmail.com>
Reported-by:  <syzbot+f9f3f388440283da2965@syzkaller.appspotmail.com>
Fixes: cbebd363

 ("batman-adv: Use own timer for multicast TT and TVLV updates")
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>