generic/251: constrain runtime via time/load/soak factors

On my test fleet, this test can run for well in excess of 20 minutes:

   613 generic/251
   616 generic/251
   624 generic/251
   630 generic/251
   634 generic/251
   652 generic/251
   675 generic/251
   749 generic/251
   777 generic/251
   808 generic/251
   832 generic/251
   946 generic/251
  1082 generic/251
  1221 generic/251
  1241 generic/251
  1254 generic/251
  1305 generic/251
  1366 generic/251
  1646 generic/251
  1936 generic/251
  1952 generic/251
  2358 generic/251
  4359 generic/251
  5325 generic/251
 34046 generic/251

because it hardcodes 20 threads and 10 copies.  It's not great to have a
test that results in a significant fraction of the total test runtime.
Fix the looping and load on this test to use LOAD and TIME_FACTOR to
scale up its operations, along with the usual SOAK_DURATION override.
That brings the default runtime down to less than a minute.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>