fuse: handle idmappings properly in ->write_iter()

This is needed to properly clear suid/sgid.

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: mihalicyn

fs/fuse/file.c

@@ -1398,6 +1398,7 @@ static void fuse_dio_unlock(struct kiocb *iocb, bool exclusive)
 static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
 {
 	struct file *file = iocb->ki_filp;
+	struct mnt_idmap *idmap = file_mnt_idmap(file);
 	struct address_space *mapping = file->f_mapping;
 	ssize_t written = 0;
 	struct inode *inode = mapping->host;
@@ -1412,7 +1413,7 @@ static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
 			return err;
 
 		if (fc->handle_killpriv_v2 &&
-		    setattr_should_drop_suidgid(&nop_mnt_idmap,
+		    setattr_should_drop_suidgid(idmap,
 						file_inode(file))) {
 			goto writethrough;
 		}