net: icmp: Add a way to send ICMP Echo-Req without waiting

Create a net_icmp_send_echo_request_no_wait() variant that will
not wait when allocating net_buf to be sent. This variant will avoid
a warning to be printed when sending Echo-Req from a system workqueue
because the work cannot sleep in this case.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>