kernel: mtd: Make subpartitions inherit parent's access mode

kernel: mtd: Make subpartitions inherit parent's access mode

Currently it's not possible to effectively mark a "firmware" partition
as read-only. The sub-partitions "kernel", "rootfs" and "rootfs_data"
are always created as read-write (ignoring the parent access mode).

This patch enforces the access mode of sub-partitions to match the
parent partition, which is useful for recovery images that are meant
to be fully read-only to avoid accidental damage from end-user.

An example of such implementation (read-only firmware image) is the
recovery image used on the Zsun-SD100 [1].

Please note the related patch for fstools [2] to enable this read-only
concept.

[1] https://github.com/brunompena/zsun-resources
[2] http://lists.infradead.org/pipermail/openwrt-devel/2020-January/021043.html

Signed-off-by: Bruno Pena <brunompena@gmail.com>
[removed already obsolete 4.9 kernel patch]
Signed-off-by: Petr Štetiar <ynezz@true.cz>