[PACKET]: Add optional checksum computation for recvmsg

This patch is needed to make ISC's DHCP server (and probably other
DHCP servers/clients using AF_PACKET) to be able to serve another
client on the same Xen host.

The problem is that packets between different domains on the same
Xen host only have partial checksums.  Unfortunately this piece of
information is not passed along in AF_PACKET unless you're using
the mmap interface.  Since dhcpd doesn't support packet-mmap, UDP
packets from the same host come out with apparently bogus checksums.

This patch adds a mechanism for AF_PACKET recvmsg(2) to return the
status along with the packet.  It does so by adding a new cmsg that
contains this information along with some other relevant data such
as the original packet length.

I didn't include the time stamp information since there is already
a cmsg for that.

This patch also changes the mmap code to set the CSUMNOTREADY flag
on all packets instead of just outoing packets on cooked sockets.

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