LU-6303 osc: remove handling cl_avail_grant less than zero

LU-6303 osc: remove handling cl_avail_grant less than zero

Earlier cl_avail_grant was changed to an unsigned int. Juila
Lawall reported for the upstream client the following which
affects the Intel branch as well:

drivers/staging/lustre/lustre/osc/osc_request.c:1045:5-24: WARNING: Unsigned
     expression compared with zero: cli -> cl_avail_grant < 0

Since cl_avail_grant can never be negative we can remove the
code handling the negative value case.

Change-Id: I10f7ac3aaab7ebf03a7f7ac0717b60134f09cddf
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: http://review.whamcloud.com/23155
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Reviewed-by: Frank Zago <fzago@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>