lib: alloc_tag_module_unload must wait for pending kfree_rcu calls

Ben Greear reports following splat:
 ------------[ cut here ]------------
 net/netfilter/nf_nat_core.c:1114 module nf_nat func:nf_nat_register_fn has 256 allocated at module unload
 WARNING: CPU: 1 PID: 10421 at lib/alloc_tag.c:168 alloc_tag_module_unload+0x22b/0x3f0
 Modules linked in: nf_nat(-) btrfs ufs qnx4 hfsplus hfs minix vfat msdos fat
...
 Hardware name: Default string Default string/SKYBAY, BIOS 5.12 08/04/2020
 RIP: 0010:alloc_tag_module_unload+0x22b/0x3f0
  codetag_unload_module+0x19b/0x2a0
  ? codetag_load_module+0x80/0x80

nf_nat module exit calls kfree_rcu on those addresses, but the free
operation is likely still pending by the time alloc_tag checks for leaks.

Wait for outstanding kfree_rcu operations to complete before checking
resolves this warning.

Reproducer:
unshare -n iptables-nft -t nat -A PREROUTING -p tcp
grep nf_nat /proc/allocinfo # will list 4 allocations
rmmod nft_chain_nat
rmmod nf_nat                # will WARN.

[akpm@linux-foundation.org: add comment]
Link: https://lkml.kernel.org/r/20241007205236.11847-1-fw@strlen.de
Fixes: a473573964 ("lib: code tagging module support")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reported-by: Ben Greear <greearb@candelatech.com>
Closes: https://lore.kernel.org/netdev/bdaaef9d-4364-4171-b82b-bcfc12e207eb@candelatech.com/
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/codetag.c

@@ -228,6 +228,9 @@ bool codetag_unload_module(struct module *mod)
 	if (!mod)
 		return true;
 
+	/* await any module's kfree_rcu() operations to complete */
+	kvfree_rcu_barrier();
+
 	mutex_lock(&codetag_lock);
 	list_for_each_entry(cttype, &codetag_types, link) {
 		struct codetag_module *found = NULL;