net: pkt: Set default data_len to iface MTU

When creating net_pkt, the default value of net_pkt data_len was
set to zero. In case of RAW sockets, when family is AF_PACKET,
the data_len will be zero as there is no higher level protocol
information. So in this case, the default value of data_len
must be set to interface MTU

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>