block: Add BDRV_REQ_NO_FALLBACK

For qemu-img convert, we want an operation that zeroes out the whole
image if this can be done efficiently, but that returns an error
otherwise so we don't write explicit zeroes and immediately overwrite
them with the real data, potentially doubling the amount of data to be
written.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Eric Blake <eblake@redhat.com>

include/block/block.h

@@ -83,8 +83,13 @@ typedef enum {
      */
     BDRV_REQ_SERIALISING        = 0x80,
 
+    /* Execute the request only if the operation can be offloaded or otherwise
+     * be executed efficiently, but return an error instead of using a slow
+     * fallback. */
+    BDRV_REQ_NO_FALLBACK        = 0x100,
+
     /* Mask of valid flags */
-    BDRV_REQ_MASK               = 0xff,
+    BDRV_REQ_MASK               = 0x1ff,
 } BdrvRequestFlags;
 
 typedef struct BlockSizes {