target_core_rd: break out unterminated loop during copy

The loop in rd_execute_rw() will never terminate if the
sg element has a zero size. Or it'll spill over into
outer space if the sg element is larger than the available
space.
So we need to add some safety catches here.

Cc: Nic Bellinger <[e-mail]>
Signed-off-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/target_core_rd.c

···
 		void *rd_addr;
 
 		sg_miter_next(&m);
+		if (!(u32)m.length) {
+			pr_debug("RD[%u]: invalid sgl %p len %zu\n",
+				 dev->rd_dev_id, m.addr, m.length);
+			sg_miter_stop(&m);
+			return TCM_INCORRECT_AMOUNT_OF_DATA;
+		}
 		len = min((u32)m.length, src_len);
+		if (len > rd_size) {
+			pr_debug("RD[%u]: size underrun page %d offset %d "
+				 "size %d\n", dev->rd_dev_id,
+				 rd_page, rd_offset, rd_size);
+			len = rd_size;
+		}
 		m.consumed = len;
 
 		rd_addr = sg_virt(rd_sg) + rd_offset;