fusefrontend: -sharedstorage: fix TestRmwRace failure

The Open() and Create() paths used different inode numbers,
which broke openfiletable locking against concurred readers.

internal/fusefrontend/node.go

@@ -130,10 +130,15 @@ func (n *Node) Create(ctx context.Context, name string, flags uint32, mode uint3
 		errno = fs.ToErrno(err)
 		return
 	}
+	// Save original stat values before newChild() translates the inode number.
+	// For an open fd, we assume the inode number cannot change behind our back,
+	// even in `-sharedstorage` mode.
+	origSt := st
+
 	ch := n.newChild(ctx, &st, out)
 
 	f := os.NewFile(uintptr(fd), cName)
-	return ch, NewFile(f, rn, &st), 0, 0
+	return ch, NewFile(f, rn, &origSt), 0, 0
 }
 
 // Unlink - FUSE call. Delete a file.

internal/fusefrontend/node_helpers.go

@@ -100,7 +100,8 @@ func (n *Node) prepareAtSyscall(child string) (dirfd int, cName string, errno sy
 }
 
 // newChild attaches a new child inode to n.
-// The passed-in `st` will be modified to get a unique inode number.
+// The passed-in `st` will be modified to get a unique inode number
+// (or, in `-sharedstorage` mode, the inode number will be set to zero).
 func (n *Node) newChild(ctx context.Context, st *syscall.Stat_t, out *fuse.EntryOut) *fs.Inode {
 	rn := n.rootNode()
 	// Get stable inode number based on underlying (device,ino) pair