xfs: set IOMAP_F_NEW more carefully

Don't set IOMAP_F_NEW if we COW over an existing allocated range, as
these aren't strictly new allocations.  This is required to be able to
use IOMAP_F_NEW to zero newly allocated blocks, which is required for
the iomap code to fully support file systems that don't do delayed
allocations or use unwritten extents.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_iomap.c

@@ -707,9 +707,12 @@ xfs_file_iomap_begin_delay(
 	 * Flag newly allocated delalloc blocks with IOMAP_F_NEW so we punch
 	 * them out if the write happens to fail.
 	 */
-	iomap_flags |= IOMAP_F_NEW;
-	trace_xfs_iomap_alloc(ip, offset, count, whichfork,
-			whichfork == XFS_DATA_FORK ? &imap : &cmap);
+	if (whichfork == XFS_DATA_FORK) {
+		iomap_flags |= IOMAP_F_NEW;
+		trace_xfs_iomap_alloc(ip, offset, count, whichfork, &imap);
+	} else {
+		trace_xfs_iomap_alloc(ip, offset, count, whichfork, &cmap);
+	}
 done:
 	if (whichfork == XFS_COW_FORK) {
 		if (imap.br_startoff > offset_fsb) {