Missing support for mount --bind of nondirectory files

### Description

mount --bind fails unless the source and destination are directories.

In Linux, any non-directory filesystem node can be bind mounted on any non-directory target (in addition to bind mounting directories on directories).

### Steps to reproduce

```
echo foo > foo
echo bar > bar
mount --bind foo bar
```

### runsc version

```shell

```

### docker version (if using docker)

```shell

```

### uname

_No response_

### kubectl (if using Kubernetes)

```shell

```

### repo state (if built from source)

_No response_

### runsc debug logs (if available)

```shell

```