bcachefs: Fix reporting of freed objects from key cache shrinker

bcachefs: Fix reporting of freed objects from key cache shrinker

We count objects as freed when we move them to the srcu-pending lists
because we're doing the equivalent of a kfree_srcu(); the only
difference is managing the pending list ourself means we can allocate
from the pending list.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>