LU-3321 obdclass: Add a preallocated percpu cl_env

LU-3321 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>
Change-Id: I14a06294f0c2caae8806d7da134a8076f75ddc81
Reviewed-on: http://review.whamcloud.com/8174
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>