net: initialize ip_buf_len when receiving packet

When udp_packet_receive is called, ip_buf_len is set 4 bytes too big.
This ends up with the client program receiving 4 bytes more than the
board actually received. Initialize it correctly alongside other values.

Change-Id: I0012aa7e8c6a440a618dfa03c4fdc2b898d592fc
Signed-off-by: Jaakko Hannikainen <jaakko.hannikainen@intel.com>