mount: Fix cleanup_mnt_ns() when opts.root is set

In commit

    c1404f66
    mount: restore cwd after creating a roots yard (v2)

were introduced changes to the function cleanup_mnt_ns() that handle
the case when opts.root is set.

However, after commit

    2e8970be
    mount: create a mount point for the root mount namespace in the roots yard

the cleanup_mnt_ns() function always results in error:

    mnt: Can't remove the directory ...: No such file or directory

when restore fails and opts.root has been set.

Resolves #467

Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>