telemetry: fix race on callbacks list

telemetry: fix race on callbacks list

The list_commands() function accessed the callbacks list,
but did not take the lock. This may have caused inconsistencies if
callbacks were being registered at the same time.
This is now fixed to lock before iterating the list,
and unlock afterwards.

Fixes: f38748736eb2 ("telemetry: add default callback commands")
Cc: stable@dpdk.org
Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Ciara Power <ciara.power@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>