pivot_root.2: Remove bogus a bogus EBUSY error case

pivot_root.2: Remove bogus a bogus EBUSY error case

The note that EBUSY is given if a filesystem is already mounted
on 'Iput_old' was never really true. That restriction was in
Linux 2.3.14, but removed in Linux 2.3.99-pre6 so it never made
it to mainline.

The relevant diff in pivot_root() was:

        error = -EBUSY;
-       if (d_new_root->d_sb == root->d_sb || d_put_old->d_sb == root->d_sb)
+       if (new_nd.mnt == root_mnt || old_nd.mnt == root_mnt)
                goto out2; /* loop */
-       if (d_put_old != d_put_old->d_covers)
-               goto out2; /* mount point is busy */
        error = -EINVAL;

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>