md/raid5: use do_div() for 64 bit divisions in raid5_sync_request

We get compilation error on 32-bit architectures (e.g. m68k), as:

  ERROR: modpost: "__udivdi3" [drivers/md/raid456.ko] undefined!

Since 'sync_blocks' is defined as u64, use do_div() to fix this error.

Fixes: c911c46c ("md/raid456: convert macro STRIPE_* to RAID5_STRIPE_*")
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>