bond: show trace causes a crash if the interface is deleted

bond: show trace causes a crash if the interface is deleted

For the debug image, if the interface is removed and the trace was
collected prior to the interface delete, show trace may cause a crash.
This is because vnet_get_sw_interface_name and vnet_get_sup_hw_interface
are not safe if the interface is deleted.

The fix is to use format_vnet_sw_if_index_name if all we need is to
get the interface name in the trace to display. It would show "DELETED"
which is better than a crash.

Change-Id: I912402d3e71592ece9f49d36c8a6b7af97f3b69e
Signed-off-by: Steven <sluong@cisco.com>