xfs: introduce the CoW fork

Introduce a new in-core fork for storing copy-on-write delalloc
reservations and allocated extents that are in the process of being
written out.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
index 476bf2b..ae59c3c 100644
--- a/fs/xfs/xfs_trace.h
+++ b/fs/xfs/xfs_trace.h
@@ -269,10 +269,10 @@
 		__field(unsigned long, caller_ip)
 	),
 	TP_fast_assign(
-		struct xfs_ifork	*ifp = (state & BMAP_ATTRFORK) ?
-						ip->i_afp : &ip->i_df;
+		struct xfs_ifork	*ifp;
 		struct xfs_bmbt_irec	r;
 
+		ifp = xfs_iext_state_to_fork(ip, state);
 		xfs_bmbt_get_all(xfs_iext_get_ext(ifp, idx), &r);
 		__entry->dev = VFS_I(ip)->i_sb->s_dev;
 		__entry->ino = ip->i_ino;