ipmr: Fix access to mfc_cache_list without lock held

ipmr: Fix access to mfc_cache_list without lock held

Accessing `mr_table->mfc_cache_list` is protected by an RCU lock. In the
following code flow, the RCU read lock is not held, causing the
following error when `RCU_PROVE` is not held. The same problem might
show up in the IPv6 code path.

	6.12.0-rc5-kbuilder-01145-gbac17284bdcb #33 Tainted: G            E    N
	-----------------------------
	net/ipv4/ipmr_base.c:313 RCU-list traversed in non-reader section!!

	rcu_scheduler_active = 2, debug_locks = 1
		   2 locks held by RetransmitAggre/3519:
		    #0: ffff88816188c6c0 (nlk_cb_mutex-ROUTE){+.+.}-{3:3}, at: __netlink_dump_start+0x8a/0x290
		    #1: ffffffff83fcf7a8 (rtnl_mutex){+.+.}-{3:3}, at: rtnl_dumpit+0x6b/0x90

	stack backtrace:
		    lockdep_rcu_suspicious
		    mr_table_dump
		    ipmr_rtm_dumproute
		    rtnl_dump_all
		    rtnl_dumpit
		    netlink_dump
		    __netlink_dump_start
		    rtnetlink_rcv_msg
		    netlink_rcv_skb
		    netlink_unicast
		    netlink_sendmsg

This is not a problem per see, since the RTNL lock is held here, so, it
is safe to iterate in the list without the RCU read lock, as suggested
by Eric.

To alleviate the concern, modify the code to use
list_for_each_entry_rcu() with the RTNL-held argument.

The annotation will raise an error only if RTNL or RCU read lock are
missing during iteration, signaling a legitimate problem, otherwise it
will avoid this false positive.

This will solve the IPv6 case as well, since ip6mr_rtm_dumproute() calls
this function as well.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20241108-ipmr_rcu-v2-1-c718998e209b@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>