staging/lustre/ptlrpc: move procfs threads* files to sysfs

Move ptlrpc service threads_min, threads_max and threads_running
entries from procfs to sysfs.
Currently in use only by ldlm callback service only in
/sys/fs/lustre/ldlm/services/ldlm_cbd/

Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>