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

Lustre-change: https://review.whamcloud.com/#/c/39827
Lustre-commit: e05ccafd6ee214895d01efbb13a3757e3625a859

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Idcd25ed427c23735e1cdc70359bace43b5b9d886
Reviewed-on: https://review.whamcloud.com/39380
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>