Don't drop the mountpoint reference in already umounted dead mountpoints.

PiperOrigin-RevId: 599952931

pkg/sentry/vfs/dentry.go

@@ -338,9 +338,11 @@ func (vfs *VirtualFilesystem) forgetDeadMountpoint(ctx context.Context, d *Dentr
 	vfs.lockMounts()
 	defer vfs.unlockMounts(ctx)
 	for mnt := range vfs.mountpoints[d] {
+		// If umounted is true, the mount point has already been decrefed by umount
+		// so we don't need to release the reference again here.
 		if mnt.umounted {
 			vfs.mounts.seq.BeginWrite()
-			vfs.delayDecRef(vfs.disconnectLocked(mnt))
+			vfs.disconnectLocked(mnt)
 			vfs.delayDecRef(mnt)
 			vfs.mounts.seq.EndWrite()
 		} else {