inet: bring NLM_DONE out to a separate recv() again

Commit under Fixes optimized the number of recv() calls
needed during RTM_GETROUTE dumps, but we got multiple
reports of applications hanging on recv() calls.
Applications expect that a route dump will be terminated
with a recv() reading an individual NLM_DONE message.

Coalescing NLM_DONE is perfectly legal in netlink,
but even tho reporters fixed the code in respective
projects, chances are it will take time for those
applications to get updated. So revert to old behavior
(for now)?

Old kernel (5.19):

 $ ./cli.py --dbg-small-recv 4096 --spec netlink/specs/rt_route.yaml \
            --dump getroute --json '{"rtm-family": 2}'
 Recv: read 692 bytes, 11 messages
   nl_len = 68 (52) nl_flags = 0x22 nl_type = 24
 ...
   nl_len = 60 (44) nl_flags = 0x22 nl_type = 24
 Recv: read 20 bytes, 1 messages
   nl_len = 20 (4) nl_flags = 0x2 nl_type = 3

Before (6.9-rc2):

 $ ./cli.py --dbg-small-recv 4096 --spec netlink/specs/rt_route.yaml \
            --dump getroute --json '{"rtm-family": 2}'
 Recv: read 712 bytes, 12 messages
   nl_len = 68 (52) nl_flags = 0x22 nl_type = 24
 ...
   nl_len = 60 (44) nl_flags = 0x22 nl_type = 24
   nl_len = 20 (4) nl_flags = 0x2 nl_type = 3

After:

 $ ./cli.py --dbg-small-recv 4096 --spec netlink/specs/rt_route.yaml \
            --dump getroute --json '{"rtm-family": 2}'
 Recv: read 692 bytes, 11 messages
   nl_len = 68 (52) nl_flags = 0x22 nl_type = 24
 ...
   nl_len = 60 (44) nl_flags = 0x22 nl_type = 24
 Recv: read 20 bytes, 1 messages
   nl_len = 20 (4) nl_flags = 0x2 nl_type = 3

Reported-by: Stefano Brivio <[email protected]>
Link: https://lore.kernel.org/all/20240315124808.033ff58d@elisabeth
Reported-by: Ilya Maximets <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Fixes: 4ce5dc9 ("inet: switch inet_dump_fib() to RCU protection")
Signed-off-by: Jakub Kicinski <[email protected]>
Tested-by: Ilya Maximets <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/ipv4/fib_frontend.c

@@ -1050,6 +1050,11 @@ static int inet_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 			e++;
 		}
 	}
+
+	/* Don't let NLM_DONE coalesce into a message, even if it could.
+	 * Some user space expects NLM_DONE in a separate recv().
+	 */
+	err = skb->len;
 out:
 
 	cb->args[1] = e;