FIX: ping_monitor() usage causes memory leaks

FIX: ping_monitor() usage causes memory leaks

When for ping_monitor() input devnum2devname() is used,
received string pointer should be passed to free() for memory release.
It is not made in several places. This use case should have function
to avoid memory leak.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>