Fix module probe failure on 32-bit systems

Previous commit 7ef5e54e2e28884a04dc800657967b891239e933 caused
module probe failure on 32-bit systems, dmesg showed
Unknown symbol __moddi3
The problem was caused by the modulo operation gethrtime() % tqs->stqs_count
in the committed code, instead of implementing __moddi3 for all 32-bit systems,
advised by Behlendorf, we can just cast the return value of gethrtime() into
a uint64_t, since gethrtime does not return negative value on all circumstances
we need not care about the potential overflow.

Signed-off-by: Ying Zhu <casualfisher@gmail.com>