net: route dump netlink NLM_F_MULTI flag missing

An excerpt from netlink(7) man page,

  In multipart messages (multiple nlmsghdr headers with associated payload
  in one byte stream) the first and all following headers have the
  NLM_F_MULTI flag set, except for the last  header  which  has the type
  NLMSG_DONE.

but, after (ee28906f) there is a missing NLM_F_MULTI flag in the middle of a
FIB dump. The result is user space applications following above man page
excerpt may get confused and may stop parsing msg believing something went
wrong.

In the golang netlink lib [0] the library logic stops parsing believing the
message is not a multipart message. Found this running Cilium[1] against
net-next while adding a feature to auto-detect routes. I noticed with
multiple route tables we no longer could detect the default routes on net
tree kernels because the library logic was not returning them.

Fix this by handling the fib_dump_info_fnhe() case the same way the
fib_dump_info() handles it by passing the flags argument through the
call chain and adding a flags argument to rt_fill_info().

Tested with Cilium stack and auto-detection of routes works again. Also
annotated libs to dump netlink msgs and inspected NLM_F_MULTI and
NLMSG_DONE flags look correct after this.

Note: In inet_rtm_getroute() pass rt_fill_info() '0' for flags the same
as is done for fib_dump_info() so this looks correct to me.

[0] https://github.com/vishvananda/netlink/
[1] https://github.com/cilium/



Fixes: ee28906f ("ipv4: Dump route exceptions if requested")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>