main/postmarketos-initramfs: mount root partition we unlocked (MR 5920)

When using an encrypted installation of postmarketOS, the pmos_root_uuid=
on the cmdline only tells us the UUID of the crypto_LUKS partition. Once
the partition is unlocked, we perform the the old unreliable auto detection
again. This might mount the wrong partition if multiple installations of
pmOS are attached to the system.

After we unlock the root partition, we know exactly where the root
partition is supposed to be (= at /dev/mapper/root). Let's use that
directly instead of going through the whole detection sequence again.