io_uring/bpf_filters: retain COW'ed settings on parse failures

io_uring/bpf_filters: retain COW'ed settings on parse failures

If io_parse_restrictions() fails, it ends up clearing any restrictions
currently set. The intent is only to clear whatever it already applied,
but it ends up clearing everything, including whatever settings may have
been applied in a copy-on-write fashion already. Ensure that those are
retained.

Link: https://lore.kernel.org/io-uring/CAK8a0jzF-zaO5ZmdOrmfuxrhXuKg5m5+RDuO7tNvtj=kUYbW7Q@mail.gmail.com/
Reported-by: antonius <bluedragonsec2023@gmail.com>
Fixes: ed82f35b926b ("io_uring: allow registration of per-task restrictions")
Signed-off-by: Jens Axboe <axboe@kernel.dk>