[JFFS2] Remove flash offset argument from various functions.

[JFFS2] Remove flash offset argument from various functions.

We don't need the upper layers to deal with the physical offset. It's
_always_ c->nextblock->offset + c->sector_size - c->nextblock->free_size
so we might as well just let the actual write functions deal with that.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>