LU-12438 llite: vfs_read/write removed, use kernel_read/write

As of Linux 4.14 the vfs_read() is no longer available
to kernel modules. The kernel_read() function calls
vfs_read() and will continue to be available.

Adding a configure test to use kernel_read() as the
function signature changed in 4.14 to match the other file I/O
helpers.

Also remove vfs_write() in favor of kernel_write() wrapper
cfs_kernel_write().

Fixes: f172b1168857 ("LU-10092 llite: Add persistent cache on client")
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I5e5fce0e6644ba750169f3bf11ac5c98525da0a7
Reviewed-on: https://review.whamcloud.com/35223
Tested-by: Jenkins
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>