net/tcp_sigpool: Use kref_get_unless_zero()

The freeing and re-allocation of algorithm are protected by cpool_mutex,
so it doesn't fix an actual use-after-free, but avoids a deserved
refcount_warn_saturate() warning.

A trivial fix for the racy behavior.

Fixes: 8c73b26 ("net/tcp: Prepare tcp_md5sig_pool for TCP-AO")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_sigpool.c

@@ -162,9 +162,8 @@ int tcp_sigpool_alloc_ahash(const char *alg, size_t scratch_size)
 		if (strcmp(cpool[i].alg, alg))
 			continue;
 
-		if (kref_read(&cpool[i].kref) > 0)
-			kref_get(&cpool[i].kref);
-		else
+		/* pairs with tcp_sigpool_release() */
+		if (!kref_get_unless_zero(&cpool[i].kref))
 			kref_init(&cpool[i].kref);
 		ret = i;
 		goto out;