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/smbfs/inode.c b/fs/smbfs/inode.c
index dfa1d67..9551cb6 100644
--- a/fs/smbfs/inode.c
+++ b/fs/smbfs/inode.c
@@ -714,7 +714,7 @@
 		error = server->ops->truncate(inode, attr->ia_size);
 		if (error)
 			goto out;
-		error = vmtruncate(inode, attr->ia_size);
+		error = simple_setsize(inode, attr->ia_size);
 		if (error)
 			goto out;
 		refresh = 1;