xfs: punch delalloc extents from the COW fork for COW writes

When ->iomap_end is called on a short write to the COW fork it needs to
punch stale delalloc data from the COW fork and not the data fork.

Ensure that IOMAP_F_NEW is set for new COW fork allocations in
xfs_buffered_write_iomap_begin, and then use the IOMAP_F_SHARED flag
in xfs_buffered_write_delalloc_punch to decide which fork to punch.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/xfs_iomap.c

@@ -1196,7 +1196,7 @@ xfs_buffered_write_iomap_begin(
 				imap.br_startoff - offset_fsb);
 	}
 
-	iomap_flags = IOMAP_F_SHARED;
+	iomap_flags |= IOMAP_F_SHARED;
 	seq = xfs_iomap_inode_sequence(ip, iomap_flags);
 	xfs_iunlock(ip, lockmode);
 	return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags, iomap_flags, seq);
@@ -1213,8 +1213,10 @@ xfs_buffered_write_delalloc_punch(
 	loff_t			length,
 	struct iomap		*iomap)
 {
-	xfs_bmap_punch_delalloc_range(XFS_I(inode), XFS_DATA_FORK, offset,
-			offset + length);
+	xfs_bmap_punch_delalloc_range(XFS_I(inode),
+			(iomap->flags & IOMAP_F_SHARED) ?
+				XFS_COW_FORK : XFS_DATA_FORK,
+			offset, offset + length);
 }
 
 static int