Do not enable stack tracer for ZFS performance test

Linux ZFS test suite runs with /proc/sys/kernel/stack_tracer_enabled=1,
via zfs.sh script, which has negative performance impact, up to 40%.

Since large stack is a rare issue now, preferred behavior would be:

• making stack tracer an opt-in feature for zfs.sh
• zfs-test.sh enables stack tracer only when requested

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Tony Nguyen <tony.nguyen@delphix.com>
#8173