IB/mthca: Return proper error codes from mthca_fmr_alloc()

If the allocation of the MTT or the mailbox failed, mthca_fmr_alloc()
would return 0 (success) no matter what. This leads to crashes a
little down the road, when we try to dereference eg mr->mtt, which was
really ERR_PTR(-Ewhatever).

Signed-off-by: Olaf Kirch <[e-mail]>
Signed-off-by: Roland Dreier <[e-mail]>

drivers/infiniband/hw/mthca/mthca_mr.c

···
 			sizeof *(mr->mem.tavor.mpt) * idx;
 
 	mr->mtt = __mthca_alloc_mtt(dev, list_len, dev->mr_table.fmr_mtt_buddy);
-	if (IS_ERR(mr->mtt))
+	if (IS_ERR(mr->mtt)) {
+		err = PTR_ERR(mr->mtt);
 		goto err_out_table;
+	}
 
 	mtt_seg = mr->mtt->first_seg * MTHCA_MTT_SEG_SIZE;
 
···
 		mr->mem.tavor.mtts = dev->mr_table.tavor_fmr.mtt_base + mtt_seg;
 
 	mailbox = mthca_alloc_mailbox(dev, GFP_KERNEL);
-	if (IS_ERR(mailbox))
+	if (IS_ERR(mailbox)) {
+		err = PTR_ERR(mailbox);
 		goto err_out_free_mtt;
+	}
 
 	mpt_entry = mailbox->buf;