block: iolatency: avoid 64-bit division

On 32-bit architectures, dividing a 64-bit number needs to use the
do_div() function or something like it to avoid a link failure:

block/blk-iolatency.o: In function `iolatency_prfill_limit':
blk-iolatency.c:(.text+0x8cc): undefined reference to `__aeabi_uldivmod'

Using div_u64() gives us the best output and avoids the need for an
explicit cast.

Fixes: d7067512 ("block: introduce blk-iolatency io controller")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iolatency.c

@@ -798,8 +798,7 @@ static u64 iolatency_prfill_limit(struct seq_file *sf,
 	if (!dname || !iolat->min_lat_nsec)
 		return 0;
 	seq_printf(sf, "%s target=%llu\n",
-		   dname,
-		   (unsigned long long)iolat->min_lat_nsec / NSEC_PER_USEC);
+		   dname, div_u64(iolat->min_lat_nsec, NSEC_PER_USEC));
 	return 0;
 }