very low IOPS due to "block: reduce kblockd_mod_delayed_work_on() CPU consumption"

[Alex Xu (Hello71)]

Hi,

I recently noticed that between 6441998e2e and 9eaa88c703, I/O became 
much slower on my machine using ext4 on dm-crypt on NVMe with bfq 
scheduler. Checking iostat during heavy usage (find / -xdev and fstrim 
-v /), maximum IOPS had fallen from ~10000 to ~100. Reverting cb2ac2912a 
("block: reduce kblockd_mod_delayed_work_on() CPU consumption") resolves 
the issue.

Thanks,
Alex.