teamd: lw_arp_ping: make buf static and avoid returning local pointer

Fixes: 672ddfd506 ("teamd: move arp ping link watcher to a separate file")
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
1.14 release

Signed-off-by: Jiri Pirko <jiri@resnulli.us>