LU-12491 obdclass: use RCU to release lu_env_item

LU-12491 obdclass: use RCU to release lu_env_item

as rhashtable_lookup_fast() is lockless and can
find just released objects.

Fixes: aa82cc8361 ("obdclass: put all service's env on the list")
Change-Id: I6ed8ccc5bb5b192eed90b55103d11b822ec90692
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35038
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>