subsys: fs: fcb: rework fcb_walk and its callback

Previously flash_area structure was the parameter for the callback
function for transferring sector location info.
Now flash_sector is used so relevant flash_area is missing for
the callback.

This patch introduces structure fcb_entry_ctx which incorporates
entries' location and relevant flash area. It is used to pass complete
information to the callback. Additional  pointer to the flash_area fcb
speeds up operations.

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