igmp: convert struct ip_sf_list to RCU

In the Linux kernel, the following vulnerability has been resolved:

igmp: convert struct ip_sf_list to RCU

Commit 23d2b94043ca ("igmp: Add ip_mc_list lock in ip_check_mc_rcu")
added spin_lock_bh(&im->lock) to ip_check_mc_rcu() to prevent a
use-after-free while iterating im->sources during concurrent deletions.

However, ip_check_mc_rcu() is called from RCU read-side critical
sections in packet receive and route lookup fast paths (e.g.
__mkroute_output(), ip_route_input_rcu(), and __udp4_lib_rcv()).

When igmpv3_send_cr() or igmpv3_send_report() holds &pmc->lock and
calls add_grec() -> igmpv3_newpack() -> ip_route_output_ports(),
an XFRM policy matching a multicast destination triggers
xfrm_tmpl_resolve_one() -> xfrm4_get_saddr() -> __mkroute_output() ->
ip_check_mc_rcu(). This attempts to acquire &im->lock while &pmc->lock
is already held on the same CPU, triggering a lockdep recursive locking
warning / deadlock.

Fix this by converting IPv4 struct ip_sf_list to RCU, mirroring the
IPv6 implementation in net/ipv6/mcast.c:

1. Add struct rcu_head to struct ip_sf_list and annotate sf_next,
sources, and tomb as __rcu pointers.
2. Use rcu_assign_pointer() and kfree_rcu() for list updates and
deletions.
3. Remove spin_lock_bh(&im->lock) from ip_check_mc_rcu() and traverse
im->sources locklessly with for_each_psf_rcu(), reading and writing
counter fields with READ_ONCE() and WRITE_ONCE().

Note: RCU conversion of /proc/net/mcfilter will be done in a
separate patch.