mtd: ftl: Use DIV_ROUND_UP()

The kernel.h macro DIV_ROUND_UP performs the computation
(((n) + (d) - 1) / (d)) but is perhaps more readable.

Signed-off-by: Arushi Singhal <arushisinghal19971997@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>