xfs: avoid infinite loop when cancelling CoW blocks after writeback failure

When we're cancelling a cow range, we don't always delete each extent
that we iterate, so we have to move icur backwards in the list to avoid
an infinite loop.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>

fs/xfs/xfs_reflink.c

@@ -611,6 +611,9 @@ xfs_reflink_cancel_cow_blocks(
 
 			/* Remove the mapping from the CoW fork. */
 			xfs_bmap_del_extent_cow(ip, &icur, &got, &del);
+		} else {
+			/* Didn't do anything, push cursor back. */
+			xfs_iext_prev(ifp, &icur);
 		}
 next_extent:
 		if (!xfs_iext_get_extent(ifp, &icur, &got))