rbd: move the detection of destination image ealier in rename op

Return earlier to avoid extra work of reading image data and omap if the
detection fails or the destination image already exists.

Signed-off-by: Zhiqiang Wang <zhiqiang.wang@intel.com>

src/librbd/internal.cc

@@ -1212,6 +1212,17 @@ namespace librbd {
       return r;
     }
 
+    r = detect_format(io_ctx, dstname, NULL, NULL);
+    if (r < 0 && r != -ENOENT) {
+      lderr(cct) << "error checking for existing image called "
+		 << dstname << ":" << cpp_strerror(r) << dendl;
+      return r;
+    }
+    if (r == 0) {
+      lderr(cct) << "rbd image " << dstname << " already exists" << dendl;
+      return -EEXIST;
+    }
+
     string src_oid =
       old_format ? old_header_name(srcname) : id_obj_name(srcname);
     string dst_oid =
@@ -1250,17 +1261,6 @@ namespace librbd {
 	last_read = outbl.rbegin()->first;
     } while (r == MAX_READ);
 
-    r = detect_format(io_ctx, dstname, NULL, NULL);
-    if (r < 0 && r != -ENOENT) {
-      lderr(cct) << "error checking for existing image called "
-		 << dstname << ":" << cpp_strerror(r) << dendl;
-      return r;
-    }
-    if (r == 0) {
-      lderr(cct) << "rbd image " << dstname << " already exists" << dendl;
-      return -EEXIST;
-    }
-
     librados::ObjectWriteOperation op;
     op.create(true);
     op.write_full(databl);