This allows identifying /dev/root by its major:minor number which
are part of /proc/self/mountinfo but aren't contained in /proc/mounts.
Also fix jffs2reset when using an ext4 overlay by adding it to the
list of filesystems allowed if the root_only parameter of
find_mount_point is set.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>