staging/lustre/obdclass: Add a preallocated percpu cl_env

This change adds support for a single preallocated cl_env per CPU
which can be used in circumstances where reschedule is not possible.
Currently this interface is only used by the ll_releasepage function.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Prakash Surya <surya1@llnl.gov>
Reviewed-on: http://review.whamcloud.com/8174
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3321


Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>