net: ping: Use right format specifier to avoid type casting

The inet_num is u16, so use %hu instead of casting it to int. And
the sk_bound_dev_if is int actually, so it needn't cast to int.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>