fuse: serialize dirops by default

Negotiate with userspace filesystems whether they support parallel readdir
and lookup.  Disable parallelism by default for fear of breaking fuse
filesystems.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 9902af79c01a ("parallel lookups: actual switch to rwsem")
Fixes: d9b3dbdcfd62 ("fuse: switch to ->iterate_shared()")
diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index ccd4971..264f07c 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -341,8 +341,10 @@
 	struct dentry *newent;
 	bool outarg_valid = true;
 
+	fuse_lock_inode(dir);
 	err = fuse_lookup_name(dir->i_sb, get_node_id(dir), &entry->d_name,
 			       &outarg, &inode);
+	fuse_unlock_inode(dir);
 	if (err == -ENOENT) {
 		outarg_valid = false;
 		err = 0;
@@ -1341,7 +1343,9 @@
 		fuse_read_fill(req, file, ctx->pos, PAGE_SIZE,
 			       FUSE_READDIR);
 	}
+	fuse_lock_inode(inode);
 	fuse_request_send(fc, req);
+	fuse_unlock_inode(inode);
 	nbytes = req->out.args[0].size;
 	err = req->out.h.error;
 	fuse_put_request(fc, req);