RDS: IB: fix 'passing zero to ERR_PTR()' warning

Fix a static code checker warning:
 net/rds/ib_frmr.c:82 rds_ib_alloc_frmr() warn: passing zero to 'ERR_PTR'

The error path for ib_alloc_mr failure should set err to PTR_ERR.

Fixes: 1659185f ("RDS: IB: Support Fastreg MR (FRMR) memory registration mode")
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Acked-by: NSantosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/rds/ib_frmr.c

@@ -61,6 +61,7 @@ static struct rds_ib_mr *rds_ib_alloc_frmr(struct rds_ib_device *rds_ibdev,
 			 pool->fmr_attr.max_pages);
 	if (IS_ERR(frmr->mr)) {
 		pr_warn("RDS/IB: %s failed to allocate MR", __func__);
+		err = PTR_ERR(frmr->mr);
 		goto out_no_cigar;
 	}