dpif-linux: Log details when a packet is lost.

Until now, when a packet was dropped in the kernel-to-user buffers, we
logged the occurrence but nothing that would allow a person reading the
log after the fact to learn why it was dropped.  This commit adds details
that identify the major sources of packets in the buffer, which should
help.

Signed-off-by: Ben Pfaff <blp@nicira.com>