staging/lustre/lnet: Dropped messages are not accounted correctly

LNET messages that are dropped are not accounted for correctly in
/proc/sys/lnet/stats. What I assume to be a simple typo is causing
drop_length to be double-counted and drop_count to never be
incremented.

Signed-off-by: Matt Ezell <ezellma@ornl.gov>
Reviewed-on: http://review.whamcloud.com/9096
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4577


Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>