staging/lustre: 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>
Reviewed-on: http://review.whamcloud.com/20254
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4257


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>