switch_root: add subroot support

switch_root: add subroot support

The current switch_root can only switch to a new root that is the root
of a mount point.

This patch adds support for "subroots", where the new root is
somewhere below a mount point. It does this by adding in a few extra
steps to chroot into the subroot after the enclosing partition has
been moved and entered.

This will be used by OLPC, who sort-of have 2 copies of Fedora stored
on a single partition under different directory trees, where the
initramfs decides which one to boot into

[kzak@redhat.com:
    - port to the current u-l-ng switch_root code
    - don't use static buffer for "dir" in get_parent_mount()]

CC: Peter Jones <pjones@redhat.com>
Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Karel Zak <kzak@redhat.com>