io_uring/af_unix: defer registered files gc to io_uring release

io_uring/af_unix: defer registered files gc to io_uring release

Instead of putting io_uring's registered files in unix_gc() we want it
to be done by io_uring itself. The trick here is to consider io_uring
registered files for cycle detection but not actually putting them down.
Because io_uring can't register other ring instances, this will remove
all refs to the ring file triggering the ->release path and clean up
with io_ring_ctx_free().

Cc: [email protected]
Fixes: 6b06314c47e1 ("io_uring: add file set registration")
Reported-and-tested-by: David Bouman <[email protected]>
Signed-off-by: Pavel Begunkov <[email protected]>
Signed-off-by: Thadeu Lima de Souza Cascardo <[email protected]>
[axboe: add kerneldoc comment to skb, fold in skb leak fix]
Signed-off-by: Jens Axboe <[email protected]>