fuse: {io-uring} Handle teardown of ring entries

fuse: {io-uring} Handle teardown of ring entries

On teardown struct file_operations::uring_cmd requests
need to be completed by calling io_uring_cmd_done().
Not completing all ring entries would result in busy io-uring
tasks giving warning messages in intervals and unreleased
struct file.

Additionally the fuse connection and with that the ring can
only get released when all io-uring commands are completed.

Completion is done with ring entries that are
a) in waiting state for new fuse requests - io_uring_cmd_done
is needed

b) already in userspace - io_uring_cmd_done through teardown
is not needed, the request can just get released. If fuse server
is still active and commits such a ring entry, fuse_uring_cmd()
already checks if the connection is active and then complete the
io-uring itself with -ENOTCONN. I.e. special handling is not
needed.

This scheme is basically represented by the ring entry state
FRRS_WAIT and FRRS_USERSPACE.

Entries in state:
- FRRS_INIT: No action needed, do not contribute to
  ring->queue_refs yet
- All other states: Are currently processed by other tasks,
  async teardown is needed and it has to wait for the two
  states above. It could be also solved without an async
  teardown task, but would require additional if conditions
  in hot code paths. Also in my personal opinion the code
  looks cleaner with async teardown.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com> # io_uring
Reviewed-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>