kill spurious reference to vmtruncate

Lots of filesystems calls vmtruncate despite not implementing the old
->truncate method.  Switch them to use simple_setsize and add some
comments about the truncate code where it seems fitting.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/adfs/inode.c b/fs/adfs/inode.c
index 0f5e309..6f850b0 100644
--- a/fs/adfs/inode.c
+++ b/fs/adfs/inode.c
@@ -322,8 +322,9 @@
 	if (error)
 		goto out;
 
+	/* XXX: this is missing some actual on-disk truncation.. */
 	if (ia_valid & ATTR_SIZE)
-		error = vmtruncate(inode, attr->ia_size);
+		error = simple_setsize(inode, attr->ia_size);
 
 	if (error)
 		goto out;