LU-12491 obdclass: add comment for rcu handling in lu_env_remove

LU-12491 obdclass: add comment for rcu handling in lu_env_remove

During the review it was pointed out why the RCU lock was dropped
in lu_env_remove() but the code itself doesn't explain why. Add
a comment giving the details why RCU locking is not needed.

Test-parameters: trivial

Change-Id: I4fd761d2e1b4adad8e970904d56cdcd057dfe7d5
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/35447
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Neil Brown <neilb@suse.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>