postmarketos-initramfs: fix device mapper path on android devices with subpartitions (MR 5632)

On android devices using subpartitions, during boot we set up the
subpartitions so the rootfs is /dev/mapper/userdata2, however during
resize_root_partition() we resolve the original block device (e.g. sda17
on the op6) and re-create the device mapper. This results in the cached
rootfs path being wrong (since after resize it's created as
/dev/mapper/sda17p2).

Cache the subpartition source device so we re-create it to be
consistent. Otherwise the filesystem resize fails and the device drops
to debug shell on first boot.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>