ubifs: sync() causes writes even if nothing is changed

[Hans J. Koch]

Running this command:

# while true ; do sync; sleep 1; done

causes two eraseblocks being erased every second, although there
are no writes to the ubifs filesystem. I hacked some printks into
my NAND driver that print page_address and column for each erase.
With that, I get this output every second:

...
[   63.701765] erase p=0x0000ae40 c=0xffffffff
[   63.706534] erase p=0xffffffff c=0xffffffff
[   63.725492] erase p=0x0000ae80 c=0xffffffff
[   63.730260] erase p=0xffffffff c=0xffffffff
...