staging/lustre: Reintroduce global env list

This reverts a patch that was merged before lustre client
was introduced into the stagign tree, so it's not in the history.

The performance dropped a lot when memory reclaim process kicked
in as ll_releasepage() was called to destroy lustre pages. It turned
out that big overhead to allocate cl_env and keys on the fly so we
have to revert this patch.

The original problem for the reverted patch would be solved in a
follow on patch instead.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-on: http://review.whamcloud.com/7888
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3321


Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>