RDMA/erdma: Fix error code in erdma_create_scatter_mtt()

The erdma_create_scatter_mtt() function is supposed to return error
pointers.  Returning NULL will lead to an Oops.

Fixes: ed10435 ("RDMA/erdma: Implement hierarchical MTT")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/1eb400d5-d8a3-4a8e-b3da-c43c6c377f86@moroto.mountain
Acked-by: Cheng Xu <chengyou@linux.alibaba.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>

drivers/infiniband/hw/erdma/erdma_verbs.c

@@ -655,7 +655,7 @@ static struct erdma_mtt *erdma_create_scatter_mtt(struct erdma_dev *dev,
 
 	mtt = kzalloc(sizeof(*mtt), GFP_KERNEL);
 	if (!mtt)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	mtt->size = ALIGN(size, PAGE_SIZE);
 	mtt->buf = vzalloc(mtt->size);