spiffs_page_object_ix_header._align is not properly zeroed

[rojer]

I am working with a flash controller that does not like it when application tried to set bits from 0 to 1. naturally, it's not possible, and most controllers just ignore it, but not this one - it raises an error for such writes, and I found that SPIFFS is a bit sloppy in this regard. In particular, the dummy _align member of spiffs_page_object_ix_header ends up with having garbage data sometimes - in particular during operations in spiffs_object_append. it should be zeroed or left at 0xff, or set to something else - in any case, it needs to have stable value.

[pellepl]

Right, the _align member should be zeroed (or something). The transition 0->1 discussion we take in #173. :)