to be able to lookup by current thread where it's too
complicated to pass env by argument.

this version has stats to see slow/fast lookups. so, in sanity-benchmark
there were 172850 fast lookups (from per-cpu cache) and 27228 slow lookups
(from rhashtable). going to see the ration in autotest's reports.

Fixes: 2339e1b3 ("LU-11483 ldlm ofd_lvbo_init() and mdt_lvbo_fill() create env")
Fixes: e02cb407

 ("LU-11164 ldlm: pass env to lvbo methods")

Change-Id: Ia760e10fa5c68e7a18284e4726d215b330fc0eed
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34566


Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>