rtnetlink: restore handling of dumpit return value in rtnl_dump_all()

For non-zero return from dumpit() we should break the loop
in rtnl_dump_all() and return the result. Otherwise, e.g.,
we could get the memory leak in inet6_dump_fib() [1]. The
pointer to the allocated struct fib6_walker there (saved
in cb->args) can be lost, reset on the next iteration.

Fix it by partially restoring the previous behavior before
commit c63586dc ("net: rtnl_dump_all needs to propagate
error from dumpit function"). The returned error from
dumpit() is still passed further.

[1]:
unreferenced object 0xffff88001322a200 (size 96):
  comm "sshd", pid 1484, jiffies 4296032768 (age 1432.542s)
  hex dump (first 32 bytes):
    00 01 00 00 00 00 ad de 00 02 00 00 00 00 ad de  ................
    18 09 41 36 00 88 ff ff 18 09 41 36 00 88 ff ff  ..A6......A6....
  backtrace:
    [<0000000095846b39>] kmem_cache_alloc_trace+0x151/0x220
    [<000000007d12709f>] inet6_dump_fib+0x68d/0x940
    [<000000002775a316>] rtnl_dump_all+0x1d9/0x2d0
    [<00000000d7cd302b>] netlink_dump+0x945/0x11a0
    [<000000002f43485f>] __netlink_dump_start+0x55d/0x800
    [<00000000f76bbeec>] rtnetlink_rcv_msg+0x4fa/0xa00
    [<000000009b5761f3>] netlink_rcv_skb+0x29c/0x420
    [<0000000087a1dae1>] rtnetlink_rcv+0x15/0x20
    [<00000000691b703b>] netlink_unicast+0x4e3/0x6c0
    [<00000000b5be0204>] netlink_sendmsg+0x7f2/0xba0
    [<0000000096d2aa60>] sock_sendmsg+0xba/0xf0
    [<000000008c1b786f>] __sys_sendto+0x1e4/0x330
    [<0000000019587b3f>] __x64_sys_sendto+0xe1/0x1a0
    [<00000000071f4d56>] do_syscall_64+0x9f/0x300
    [<000000002737577f>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
    [<0000000057587684>] 0xffffffffffffffff

Fixes: c63586dc ("net: rtnl_dump_all needs to propagate error from dumpit function")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -3367,7 +3367,7 @@ static int rtnl_dump_all(struct sk_buff *skb, struct netlink_callback *cb)
 			cb->seq = 0;
 		}
 		ret = dumpit(skb, cb);
-		if (ret < 0)
+		if (ret)
 			break;
 	}
 	cb->family = idx;