dpif-linux: Report dropped lost messages at WARN level.

Messages about packets being lost are logged at level WARN, but when
they were generated at a high rate, those consolidated messages were
logged at ERR.  This changes to consolidated messages to be logged at
WARN, too.

Thanks to Ben Pfaff for quickly suggesting the culprit.

Bug #14783

Reported-by: James Schmidt <jschmidt@nicira.com>
Signed-off-by: Justin Pettit <jpettit@nicira.com>