dpif-netlink: Allow MRU packet attribute.

dpif-netlink: Allow MRU packet attribute.

User space now may receive re-assembled IP fragments. The user space
netlink handler can now accept packets with the new OVS_PACKET_ATTR_MRU
attribute. This allows the kernel to assemble fragmented packets for the
duration of OpenFlow processing, then re-fragment at output time. Most
notably this occurs for packets that are sent through the connection
tracker.

Note that the MRU attribute is not exported at the OpenFlow layer. As
such, if packets are reassembled by conntrack and subsequently sent to
the controller, then OVS has no way to re-serialize the packets to their
original size.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>