Fix usages of mountinfo.PrefixFilter

It says: The prefix path **must be absolute, have all symlinks resolved, and cleaned**. But those requirements are violated in lots of places.

What happens when it is given a non-canonicalized path is that `mountinfo.GetMounts` will not find mounts.

The trivial case is:
```
$ mkdir a && ln -s a b && mkdir b/c b/d && mount --bind b/c b/d && cat /proc/mounts | grep -- '[ab]/d'
/dev/sdd3 /home/user/a/d ext4 rw,noatime,discard 0 0
```
We asked to bind-mount b/c to b/d, but ended up with mount in a/d.
So, mount table always contains canonicalized mount points, and it is an error to look for non-canonicalized paths in it.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>

mount/mount.go

@@ -18,6 +18,7 @@ package mount
 
 import (
 	"fmt"
+	"path/filepath"
 	"strings"
 
 	"github.com/containerd/containerd/api/types"
@@ -69,6 +70,18 @@ func UnmountMounts(mounts []Mount, target string, flags int) error {
 	return nil
 }
 
+// CanonicalizePath makes path absolute and resolves symlinks in it.
+// Path must exist.
+func CanonicalizePath(path string) (string, error) {
+	// Abs also does Clean, so we do not need to call it separately
+	path, err := filepath.Abs(path)
+	if err != nil {
+		return "", err
+	}
+
+	return filepath.EvalSymlinks(path)
+}
+
 // ReadOnly returns a boolean value indicating whether this mount has the "ro"
 // option set.
 func (m *Mount) ReadOnly() bool {