bcachefs: Fix a 64 bit divide

this fixes builds on 32 bit.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/buckets.c

@@ -321,7 +321,7 @@ static u64 reserve_factor(u64 r)
 
 static u64 avail_factor(u64 r)
 {
-	return (r << RESERVE_FACTOR) / ((1 << RESERVE_FACTOR) + 1);
+	return div_u64(r << RESERVE_FACTOR, (1 << RESERVE_FACTOR) + 1);
 }
 
 u64 bch2_fs_sectors_used(struct bch_fs *c, struct bch_fs_usage_online *fs_usage)