ref_tracker: remove filter_irq_stacks() call

After commit e9400660 ("lib/stackdepot: always do filter_irq_stacks()
in stack_depot_save()") it became unnecessary to filter the stack
before calling stack_depot_save().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>