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: ed10435d ("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>