ubifs: add option to specify version for new file systems

Instead of creating ubifs file systems with UBIFS_FORMAT_VERSION
by default, add a module parameter ubifs.default_version to allow
the user to specify the desired version. Valid values are 4 to
UBIFS_FORMAT_VERSION (currently 5).

This way, one can for example create a file system with version 4
on kernel 4.19 which can still be mounted rw when downgrading to
kernel 4.9.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>