init.rc: Create /storage mountpoint so Dalvik can mark as slave in zygotes

Mount a tmpfs as /storage in order to create a mountpoint, so that Dalvik
can later mark /storage as slave in zygotes.

Must mount in early-init as some devices' init.hardware.rc populate
/storage then, which the tmpfs would overwrite if mounted later.

Must use tmpfs as rootfs can't be bind mounted (otherwise "mount rootfs
/storage /storage bind" would work as an alternative).