LU-12368 obdclass: don't send multiple statfs RPCs

LU-12368 obdclass: don't send multiple statfs RPCs

If multiple threads are racing to send a non-cached OST_STATFS or
MDS_STATFS RPC, this can cause a significant RPC storm for systems
with many-core clients and many OSTs due to amplification of the
requests, and the fact that STATFS RPCs are sent asynchronously.
Some logs have shown few 96-core clients have 20k+ OST_STATFS RPCs
in flight concurrently, which can overload the network if many OSTs
are on the same OSS nodes (osc.*.max_rpcs_in_flight is per OST).

This was not previously a significant issue when core counts were
smaller on the clients, or with fewer OSTs per OSS.

If a thread can't use the cached statfs values, limit statfs to one
thread at a time, since the thread(s) would be blocked waiting for
the RPC replies anyway, which can't finish faster if many are sent.

Also add a llite.*.statfs_max_age parameter that can be tuned on
to control the maximum age (in seconds) of the statfs cache.  This
can avoid overhead for workloads that are statfs heavy, given that
the filesystem is _probably_ not running out of space this second,
and even so "statfs" does not guarantee space in parallel workloads.

Lustre-change: https://review.whamcloud.com/35380
Lustre-commit: 1c41a6ac390bf74a135861efcd576a3b433d3c49

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I95690e37aecbac08ac5768a5e5c6c70ca258a832
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/35485
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>