bcachefs: fix the count of nr_freed_pcpu after changing bc->freed_nonpcpu list

bcachefs: fix the count of nr_freed_pcpu after changing bc->freed_nonpcpu list

When allocating bkey_cached from bc->freed_pcpu list, it missed
decreasing the count of nr_freed_pcpu which would cause the mismatch
between the value of nr_freed_pcpu and the list items. This problem
also exists in moving new bkey_cached to bc->freed_pcpu list.
If these happened, the bug info may appear in
bch2_fs_btree_key_cache_exit by the follow code:

   BUG_ON(list_count_nodes(&bc->freed_pcpu) != bc->nr_freed_pcpu);
   BUG_ON(list_count_nodes(&bc->freed_nonpcpu) != bc->nr_freed_nonpcpu);

Fixes: c65c13f0eac6 ("bcachefs: Run btree key cache shrinker less aggressively")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>