generic/476: fix fsstress process management

generic/476: fix fsstress process management

generic/476 never used to run fsstress in the background, but
8973af00ec212f made it do that.  This is incorrect, because now 476 runs
for three seconds (i.e. long enough to fall out the bottom of the test
and end up in _cleanup), ignoring any SOAK_DURATION/TIME_FACTOR
settings.

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>