ipnetns: enable to dump nsid conversion table

ipnetns: enable to dump nsid conversion table

This patch enables to dump/get nsid from a netns into another netns.

Example:
$ ./test.sh
+ ip netns add foo
+ ip netns add bar
+ touch /var/run/netns/init_net
+ mount --bind /proc/1/ns/net /var/run/netns/init_net
+ ip netns set init_net 11
+ ip netns set foo 12
+ ip netns set bar 13
+ ip netns
init_net (id: 11)
bar (id: 13)
foo (id: 12)
+ ip -n foo netns set init_net 21
+ ip -n foo netns set foo 22
+ ip -n foo netns set bar 23
+ ip -n foo netns
init_net (id: 21)
bar (id: 23)
foo (id: 22)
+ ip -n bar netns set init_net 31
+ ip -n bar netns set foo 32
+ ip -n bar netns set bar 33
+ ip -n bar netns
init_net (id: 31)
bar (id: 33)
foo (id: 32)
+ ip netns list-id target-nsid 12
nsid 21 current-nsid 11 (iproute2 netns name: init_net)
nsid 22 current-nsid 12 (iproute2 netns name: foo)
nsid 23 current-nsid 13 (iproute2 netns name: bar)
+ ip -n foo netns list-id target-nsid 21
nsid 11 current-nsid 21 (iproute2 netns name: init_net)
nsid 12 current-nsid 22 (iproute2 netns name: foo)
nsid 13 current-nsid 23 (iproute2 netns name: bar)
+ ip -n bar netns list-id target-nsid 33 nsid 32
nsid 32 current-nsid 32 (iproute2 netns name: foo)
+ ip -n bar netns list-id target-nsid 31 nsid 32
nsid 12 current-nsid 32 (iproute2 netns name: foo)
+ ip netns list-id nsid 13
nsid 13 (iproute2 netns name: bar)

CC: Petr Oros <poros@redhat.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Tested-by: Petr Oros <poros@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>