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: YueHaibing <yuehaibing@huawei.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>