[ATM]: Check IP header validity in mpc_send_packet

Al went through the ip_fast_csum callers and found this piece of code
that did not validate the IP header.  While root crashing the machine
by sending bogus packets through raw or AF_PACKET sockets isn't that
serious, it is still nice to react gracefully.

This patch ensures that the skb has enough data for an IP header and
that the header length field is valid.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>