net: ip: resolve NET_IPV4_FRAGMENT build warning

warning: format '%d' expects argument of type 'int', but argument 3 has
type 'size_t' {aka 'long unsigned int'} [-Wformat=]

Change the printf modifier to %zd for size_t variable will resolve the
warning.

Signed-off-by: Vijayakannan Ayyathurai <vijayakannan.ayyathurai@intel.com>