LU-10560 libcfs: Use kernel_write when appropriate

LU-10560 libcfs: Use kernel_write when appropriate

Changes in the upstream kernel might have removed
vfs_write() in favor of kernel_write().

Unfortunately, the kernel_write() was initially exported
with an API that is not plug compatible with vfs_write()

The ring down is:
- kernel_write new API
- vfs_write

Change-Id: I67f73786308561dc42b06d51c26bfb94021b7589
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-on: https://review.whamcloud.com/31154
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: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>