LU-10560 libcfs: Fix inconsistent declaration for cfs_kernel_write()

LU-10560 libcfs: Fix inconsistent declaration for cfs_kernel_write()

The header file buffer should be const void * to match the
implementation.

Test-Parameters: trivial

Change-Id: I3a09b84e4c3dbe39870be04ae98c13e9e6221a6d
Fixes: b9a32054600a8d63948cced361191aa6ae7ea8f2
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-on: https://review.whamcloud.com/31948
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>