gpt-auto: propagate gpt partition ro/rw flag into root mount

This ensures that the read/write state of the root mount matches the
read/write flag in the GPT partition table entry.

This is only used as fallback in case no ro/rw flag is specified on the
kernel cmdline, and there's no entry for the root partition in
/etc/fstab.

This is missing functionality of the GPT auto logic, as without this the
root partition was always mounted read-only — when booting with zero
configuration in /etc/fstab and /proc/cmdline —, as we defaulted to
read-only behaviour for all mounts. Moreover we honoured the r/o flag in
the partition table for all other partition types, except for the root
partition.