RDMA/erdma: Fix NULL pointer access in regmr_cmd

Fix the crash of regmr_cmd called by erdma_ib_alloc_mr. The reason is
that mr->mem.mtt is not initialized but it is accessed in regmr_cmd.

The call trace information:

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 <...>
 RIP: 0010:regmr_cmd+0x170/0x1c0 [erdma]
 <...>
Call Trace:
 ? __die+0x20/0x70
 ? page_fault_oops+0x66/0x150
 ? do_user_addr_fault+0x61/0x660
 ? exc_page_fault+0x65/0x140
 ? asm_exc_page_fault+0x22/0x30
 ? regmr_cmd+0x170/0x1c0 [erdma]
 ? preempt_count_add+0x70/0xa0
 ? _raw_spin_lock_irqsave+0x19/0x50
 ? _raw_spin_unlock_irqrestore+0x1b/0x40
 ? erdma_alloc_idx+0x51/0x90 [erdma]
 erdma_get_dma_mr+0xa3/0x120 [erdma]
 __ib_alloc_pd+0xeb/0x1c0 [ib_core]

Fixes: 7244b4aa ("RDMA/erdma: Refactor the storage structure of MTT entries")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/3d140c1d-524a-4dbe-a51c-aee4f7ecafdb@moroto.mountain/Signed-off-by: Cheng Xu <chengyou@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230908060559.80203-1-chengyou@linux.alibaba.comSigned-off-by: Leon Romanovsky <leon@kernel.org>

drivers/infiniband/hw/erdma/erdma_verbs.c

@@ -133,8 +133,8 @@ static int create_qp_cmd(struct erdma_ucontext *uctx, struct erdma_qp *qp)
 static int regmr_cmd(struct erdma_dev *dev, struct erdma_mr *mr)
 {
 	struct erdma_pd *pd = to_epd(mr->ibmr.pd);
+	u32 mtt_level = ERDMA_MR_MTT_0LEVEL;
 	struct erdma_cmdq_reg_mr_req req;
-	u32 mtt_level;
 
 	erdma_cmdq_build_reqhdr(&req.hdr, CMDQ_SUBMOD_RDMA, CMDQ_OPCODE_REG_MR);
 
@@ -147,10 +147,9 @@ static int regmr_cmd(struct erdma_dev *dev, struct erdma_mr *mr)
 			req.phy_addr[0] = sg_dma_address(mr->mem.mtt->sglist);
 			mtt_level = mr->mem.mtt->level;
 		}
-	} else {
+	} else if (mr->type != ERDMA_MR_TYPE_DMA) {
 		memcpy(req.phy_addr, mr->mem.mtt->buf,
 		       MTT_SIZE(mr->mem.page_cnt));
-		mtt_level = ERDMA_MR_MTT_0LEVEL;
 	}
 
 	req.cfg0 = FIELD_PREP(ERDMA_CMD_MR_VALID_MASK, mr->valid) |