core: Account for 64-bit overflow in open-coded div_u64_u64

core: Account for 64-bit overflow in open-coded div_u64_u64

We run into an endless loop when trying to divide something which has
bit 63 set. In this case, tmp_div will overrun and never become larger
than the dividend. Detect this case and terminate the inner loop
properly.

Reported-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>