LU-2513 osc: compute grant targets in bytes

LU-2513 osc: compute grant targets in bytes

In osc_shrink_grant() and osc_shrink_grant_to_target() convert page
unit target values to bytes before comparing to cl_avail_grant.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ie8e6e8b4b3245efa3b14777608f3a48bbab7e4e2
Reviewed-on: http://review.whamcloud.com/5495
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>