bcachefs: Replace div_u64 with div64_u64 where second param is u64

Bcachefs often uses this function to divide by nanosecond times - which
can easily cause problems when cast to u32.  For example, `cat
/sys/fs/bcachefs/*/internal/rebalance_status` would return invalid data
in the `duration waited` field because dividing by the number of
nanoseconds in a minute requires the divisor parameter to be u64.

Signed-off-by: Reed Riley <reed@riley.engineer>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>