nilfs2: add mount option that reduces super block writes

This patch introduces a mount option bad_ftl that disables the
periodic overwrites of the super block to make the file system better
suitable for bad flash memory with a bad FTL. The super block is only
written at umount time. So if there is a unclean shutdown the file
system needs to be recovered by a linear scan of all segment summary
blocks.

The linear scan is only necessary if the file system wasn't umounted
properly. So the normal mount time is not affected.

Signed-off-by: Andreas Rohner <andreas.rohner@gmx.net>