LU-12120 grants: prevent negative ted_grant value

LU-12120 grants: prevent negative ted_grant value

Add check in tgt_grant_shrink() to protect ted_grant
against negative value.

Lustre-change: https://review.whamcloud.com/34996
Lustre-commit: 7e08317ef5cbed5cd587017cbe343eb4cc52822c

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Iddea86f052124413ac60f5d0f26bcb68e376ede5
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35084
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>