net: icmpv4: Return ENETUNREACH when IPv4 is unavailable

net_icmpv4_send_echo_request currently returns EINVAL (invalid
argument) when IPv4 is unavailable.

Since the availability of IPv4 has nothing to do with the arguments
provided to this function and the meaning of EINVAL in this case is
ambiguous, return the ENETUNREACH (network is unreachable) error
number instead.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>