fs: fcb: Increase temporary buffer in fcb_append

Increase temporary buffer size to 8 bytes in fcb_append to prevent
stack overflow in case flash alignment is bigger then 2 bytes.

Signed-off-by: Jan Van Winkel <jan.van_winkel@dxplore.eu>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>