fuse: fix truncate after open

Commit e1181ee6 "vfs: pass struct file to do_truncate on O_TRUNC
opens" broke the behavior of open(O_TRUNC|O_RDONLY) in fuse.  Fuse
assumed that when called from open, a truncate() will be done, not an
ftruncate().

Fix by restoring the old behavior, based on the ATTR_OPEN flag.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/dir.c

@@ -1283,8 +1283,11 @@ static int fuse_do_setattr(struct dentry *entry, struct iattr *attr,
 	if (err)
 		return err;
 
-	if ((attr->ia_valid & ATTR_OPEN) && fc->atomic_o_trunc)
+	if (attr->ia_valid & ATTR_OPEN) {
+		if (fc->atomic_o_trunc)
 			return 0;
+		file = NULL;
+	}
 
 	if (attr->ia_valid & ATTR_SIZE)
 		is_truncate = true;