LU-13763 osc: don't allow negative grants

LU-13763 osc: don't allow negative grants

Add check in the osc_init_grant() to prevent possible
underflow of cl_avail_grant and report error if it happens

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Idcd25ed427c23735e1cdc70359bace43b5b9d886
Reviewed-on: https://review.whamcloud.com/39827
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>