mtd: sharpslpart: fix overflow on block_adr calculation

Multiplying block_num and mtd->erasesize may potentially overflow
as they are both unsigned ints and so the multiplication is evaluated
in unsigned int arithmetic.  Cast block_adr to off_t to ensure
multiplication is off_t sized to avoid any potential overflow.

Detected by CoverityScan, CID#1461264 ("Unintentional integer overflow")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>