deadlock under high i/o load (e.g. hammer reblock)

[Rumko]

Almost latest master (last commit from upstream should be
0e84c0db0d0bcd6d681bc30fd16e96c26bb92db2).

Whenever nightly cron runs and a few minutes after hammer cleanup starts doing
it's job, the machine becomes mostly unresponsive (most processes and most ssh
connections stop responding, but strangely pf still continues doing it's job).

Fortunately debugging through serial console still worked, so the dump is
available at leaf:~rumko/crash/deadlock/*.0 .
-- 
Please do not CC me, since I already receive everything from these MLs.

Regards,
Rumko