LU-8066 mdc: move mdc-specific procfs files to sysfs

LU-8066 mdc: move mdc-specific procfs files to sysfs

This moves max_rpcs_in_flight and max_pages_per_rpc to
/sys/fs/lustre/mdc/.../

Linux-commit : 2ee26222d497cf75eb9a02f324dee8b6b16e1e67

Add better error handling.

Change-Id: If6b50d2a3ef26325df0018ffb990a74ce2b75c10
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/30961
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>