io_uring: fix locked_free_list caches_free()

Don't forget to zero locked_free_nr, it's not a disaster but makes it
attempting to flush it with extra locking when there is nothing in the
list. Also, don't traverse a potentially long list freeing requests
under spinlock, splice the list and do it afterwards.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>