ip: address: fix stats64 JSON object name

ip: address: fix stats64 JSON object name

The JSON object name for statistics in ip link show is "stats644".
Looks like a typo, commit d0e720111aad ("ip: ipaddress.c: add support
for json output") contains an example with the expected "stats64" name.

The fact that no one has noticed until now is probably an indication
that no one is using this object.  Hopefully it's not too late to fix
this, although IIUC this has already been in 4.13 and 4.14 releases :S

Fixes: d0e720111aad ("ip: ipaddress.c: add support for json output")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>