kernel: ubifs: create use file system format 4 by default

kernel: ubifs: create use file system format 4 by default

Instead of creating an ubifs file system with format version 5 by
default on empty UBI volumes use the older format 4 by default. This
will make it possible to mount these file systems also on older kernel
versions.

When a user wants to do a sysupgrade from kernel 4.14 to kernel 4.9 the
old kernel has to read the file system created by the more recent kernel
which currently does not work for ubifs.

This fixes the problem by creating file systems which are compatible
with older kernel versions by default.
Kernel 4.14 will still be able to read and write UBI FS file system
version 5, it will just not be used when a ubifs partition is created
implicitly on an empty UBI volume.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>