LU-8895 target: limit grant allocation

LU-8895 target: limit grant allocation

tgt_grant_alloc() is missing a check for amount of space already
granted to a client. If the client submits number of RPCs
simultaneously when the client's grant is below its max amount of
grants then the server may grant the client with amount of grants
substantially exceeding the amount of grants requested in one RPC. In
case of decent number of clients that may lead to ENOSPC long before
the lack of disk space is really achieved.

Limit grants given to a client to asked amount plus grants for 2 full
write RPCs.

A test to illustrate the issue is included.
The test needs to lower debug level so that dd provided sufficient I/O
throughput.

Signed-off-by: Vladimir Saveliev <c17830@cray.com>
Seagate-bug-id: MRP-4013
Change-Id: Ie6a8abbad28a06bc1d55ff2fd042b9664a29e9e4
Reviewed-on: https://review.whamcloud.com/24096
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>