LU-4257 obdclass: Get rid of cl_env hash table

LU-4257 obdclass: Get rid of cl_env hash table

cl_env hash table is under heavy contention when there are lots of
processes doing IO at the same time;
reduce lock contention by replacing cl_env cache with percpu array;
remove cl_env_nested_get() and cl_env_nested_put();
remove cl_env_reenter() and cl_env_reexit();

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I04137c93e2d48f21298fe954a1c2afe1006f44d5
Reviewed-on: http://review.whamcloud.com/20254
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>