f2fs: fix 32-bit build

The addition of multiple-device support broke CONFIG_BLK_DEV_ZONED
on 32-bit machines because of a 64-bit division:

fs/f2fs/f2fs.o: In function `__issue_discard_async':
extent_cache.c:(.text.__issue_discard_async+0xd4): undefined reference to `__aeabi_uldivmod'

Unfortunately, the sector number is usually a 64-bit number, and
we guarantee that bdev_zone_size() returns a power-of-two number.

Fixes: 792b84b74b54 ("f2fs: support multiple devices")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>