btrfs: scrub: clean up division in __scrub_mark_bitmap

Use proper helpers for 64bit division and then cast to narrower type.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -2620,7 +2620,8 @@ static inline void __scrub_mark_bitmap(struct scrub_parity *sparity,
 				       u64 start, u64 len)
 {
 	u64 offset;
-	int nsectors;
+	u64 nsectors64;
+	u32 nsectors;
 	int sectorsize = sparity->sctx->fs_info->sectorsize;
 
 	if (len >= sparity->stripe_len) {
@@ -2631,7 +2632,10 @@ static inline void __scrub_mark_bitmap(struct scrub_parity *sparity,
 	start -= sparity->logic_start;
 	start = div64_u64_rem(start, sparity->stripe_len, &offset);
 	offset = div_u64(offset, sectorsize);
-	nsectors = (int)len / sectorsize;
+	nsectors64 = div_u64(len, sectorsize);
+
+	ASSERT(nsectors64 < UINT_MAX);
+	nsectors = (u32)nsectors64;
 
 	if (offset + nsectors <= sparity->nsectors) {
 		bitmap_set(bitmap, offset, nsectors);