net: shell: Fix unexpected timeout on loopback ping

In case ping is sent to own address, the request is looped back to the
stack and served before ping work had a chance to reschedule. In result,
when the final ping reply has been server, and ping operation finalized
with `ping_done()`, the work was rescheduled one last time, causing the
ping timeout to be reported. Fix this by rescheduling the work before
sending the actual request, so that the reply handler can cancel the
work properly in such case.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>