LU-9372 ptlrpc: allow to limit number of service's rqbds

LU-9372 ptlrpc: allow to limit number of service's rqbds

This patch provides a way to limit the number of rqbds per
service.
This should help to avoid OOM during heavy clients
requests load, like during target failover/recovery for
thousands of Clients.
This change has been required, even after first patch for
LU-9372 (ptlrpc: drain "ptlrpc_request_buffer_desc" objects)
which already allowed to drain unused rqbds previously
allocated during heavy load, but was not efficient during
too long period of load.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Ib43f3e07741b9fcecdfae24a3753128a939d2196
Reviewed-on: https://review.whamcloud.com/29064
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>