LU-6864 osp: manage number of modify RPCs in flight

LU-6864 osp: manage number of modify RPCs in flight

Currently we use a rpc_lock to ensure concurrent in-flight
request are handled serially to prevent the execution status
from being over written. This patch changes the osp component
to send multiple modify RPCs in parallel to the MDT. This will
improve metadata performance of cross-MDT operations.

For testing replace mkdirmany with createmany -d which does the
same thing.

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Change-Id: Icb601afabd6767463634a4c7943ec4206bc758ec
Reviewed-on: https://review.whamcloud.com/14375
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>