bcachefs: Fix sysfs rebalance duration waited formatting

cat /sys/fs/bcachefs/*/internal/rebalance_status
waiting
  io wait duration:  13.5 GiB
  io wait remaining: 627 MiB
  duration waited:   1392 m

duration waited was increasing at a rate of about 14 times the expected
rate.

div_u64 takes a u32 divisor, but u->nsecs (from time_units[]) can be
bigger than u32.

Signed-off-by: Feiko Nanninga <feiko.nanninga@fnanninga.de>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>