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_unix.go

@@ -30,6 +30,12 @@ func UnmountRecursive(target string, flags int) error {
 	if target == "" {
 		return nil
 	}
+
+	target, err := CanonicalizePath(target)
+	if err != nil {
+		return err
+	}
+
 	mounts, err := mountinfo.GetMounts(mountinfo.PrefixFilter(target))
 	if err != nil {
 		return err