generic/650: revert SOAK DURATION changes

generic/650: revert SOAK DURATION changes

Prior to commit 8973af00ec21, in the absence of an explicit
SOAK_DURATION, this test would run 2500 fsstress operations each of ten
times through the loop body.  On the author's machines, this kept the
runtime to about 30s total.  Oddly, this was changed to 30s per loop
body with no specific justification in the middle of an fsstress process
management change.

On the author's machine, this explodes the runtime from ~30s to 420s.
Put things back the way they were.

Cc: <fstests@vger.kernel.org> # v2024.12.08
Fixes: 8973af00ec212f ("fstests: cleanup fsstress process management")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>