fuzzy: do not set _FSSTRESS_PID when exercising fsx

fuzzy: do not set _FSSTRESS_PID when exercising fsx

If we're not running fsstress as the scrub exerciser, don't set
_FSSTRESS_PID because the _kill_fsstress call in the cleanup function
will think that it has to wait for a nonexistant fsstress process.
This fixes the problem of xfs/565 runtime increasing from 30s to 800s
because it tries to kill a nonexistent "565.fsstress" process and then
waits for the fsx loop control process, which hasn't been sent any
signals.

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>