ipv4: igmp: Fix potential memory leaks in igmp_mod_timer() and igmp_stop_timer()

ipv4: igmp: Fix potential memory leaks in igmp_mod_timer() and igmp_stop_timer()

When a timer is deleted and not re-armed in igmp_mod_timer(), or stopped
in igmp_stop_timer(), the code currently decrements the reference counter
of the multicast list entry @im using refcount_dec(&im->refcnt).

However, both functions can be called from the RCU reader path:
- igmp_mod_timer() via igmp_heard_query() -> for_each_pmc_rcu()
- igmp_stop_timer() via igmp_rcv() -> igmp_heard_report()

If the group im was concurrently removed from the list by ip_mc_dec_group(),
its reference count might have already been decremented to 1.

In this case, timer_delete() succeeds, and refcount_dec() decrements
the refcount from 1 to 0. Since refcount_dec() does not free the object
when it hits 0 (unlike ip_ma_put()), the im structure is leaked.

Fix this by using ip_ma_put(im) instead of refcount_dec(&im->refcnt),
and deferring the put until after the spinlock is released.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260705181756.963063-4-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>