Fix use-after-free in taskq_seq_show_impl

Fix use-after-free in taskq_seq_show_impl

taskq_seq_show_impl walks the tq_active_list to show the tqent_func and
tqent_arg. However for taskq_dispatch_ent, it's very likely that the
task entry will be freed during the function call, and causes a
use-after-free bug.

To fix this, we duplicate the task entry to an on-stack struct, and
assign it instead to tqt_task. This way, the tq_lock alone will
guarantee its safety.

Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #638
Closes #640