fib_rules: fix fib rule dumps across multiple skbs

fib_rules: fix fib rule dumps across multiple skbs

[ Upstream commit 41fc014332d91ee90c32840bf161f9685b7fbf2b ]

dump_rules returns skb length and not error.
But when family == AF_UNSPEC, the caller of dump_rules
assumes that it returns an error. Hence, when family == AF_UNSPEC,
we continue trying to dump on -EMSGSIZE errors resulting in
incorrect dump idx carried between skbs belonging to the same dump.
This results in fib rule dump always only dumping rules that fit
into the first skb.

This patch fixes dump_rules to return error so that we exit correctly
and idx is correctly maintained between skbs that are part of the
same dump.

Signed-off-by: Wilson Kok <wkok@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2:
 - s/portid/pid/
 - Check whether fib_nl_fill_rule() returns < 0, as it may return > 0 on
   success (thanks to Roland Dreier)]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Roland Dreier <roland@purestorage.com>