subsys: fs: fcb: remove f_area_id from fcb structure

It is faster to operate directly on flash_area pointer instead
of fetch it all the time using the fcb flash area id.
Also as f_area_id was needed only for get appropriate flash_area
pointer, so it is better to pass it only while initialization
the fcb and not store it in fcb instance data at all.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>