xfs: support allocating delayed extents in CoW fork

Modify xfs_bmap_add_extent_delay_real() so that we can convert delayed
allocation extents in the CoW fork to real allocations, and wire this
up all the way back to xfs_iomap_write_allocate().  In a subsequent
patch, we'll modify the writepage handler to call this.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 4a28fa9..007a520 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -362,9 +362,11 @@
 
 	if (type == XFS_IO_DELALLOC &&
 	    (!nimaps || isnullstartblock(imap->br_startblock))) {
-		error = xfs_iomap_write_allocate(ip, offset, imap);
+		error = xfs_iomap_write_allocate(ip, XFS_DATA_FORK, offset,
+				imap);
 		if (!error)
-			trace_xfs_map_blocks_alloc(ip, offset, count, type, imap);
+			trace_xfs_map_blocks_alloc(ip, offset, count, type,
+					imap);
 		return error;
 	}