writeback: fix negative bdi max pause

Toralf runs trinity on UML/i386.  After some time it hangs and the last
message line is

BUG: soft lockup - CPU#0 stuck for 22s! [trinity-child0:1521]

It's found that pages_dirtied becomes very large.  More than 1000000000
pages in this case:

period = HZ * pages_dirtied / task_ratelimit;
BUG_ON(pages_dirtied > 2000000000);
BUG_ON(pages_dirtied > 1000000000);      <---------

UML debug printf shows that we got negative pause here:

ick: pause : -984
ick: pages_dirtied : 0
ick: task_ratelimit: 0

pause:
+       if (pause < 0)  {
+               extern int printf(char *, ...);
+               printf("ick : pause : %li\n", pause);
+               printf("ick: pages_dirtied : %lu\n", pages_dirtied);
+               printf("ick: task_ratelimit: %lu\n", task_ratelimit);
+               BUG_ON(1);
+       }
trace_balance_dirty_pages(bdi,

Since pause is bounded by [min_pause, max_pause] where min_pause is also
bounded by max_pause.  It's suspected and demonstrated that the
max_pause calculation goes wrong:

ick: pause : -717
ick: min_pause : -177
ick: max_pause : -717
ick: pages_dirtied : 14
ick: task_ratelimit: 0

The problem lies in the two "long = unsigned long" assignments in
bdi_max_pause() which might go negative if the highest bit is 1, and the
min_t(long, ...) check failed to protect it falling under 0.  Fix all of
them by using "unsigned long" throughout the function.

idea is taken here -
https://github.com/flar2/ElementalX-N5/commit/28325b42905bd3acc9c7088c9c4266156f7ee06f