io_uring/openclose: fix io_pipe_fixed() slot tracking for specific slots

io_uring/openclose: fix io_pipe_fixed() slot tracking for specific slots

__io_fixed_fd_install() returns 0 on success for non-alloc mode
(specific slot), not the slot index. io_pipe_fixed() used this return
value directly as the slot index in fds[], which can cause the reported
values returned via copy_to_user() to be incorrect, or the error path
operating on the incorrect direct descriptor.

Fix by computing the actual 0-based slot index (slot - 1) for specific
slot mode, while preserving the existing behavior for auto-alloc mode
where __io_fixed_fd_install() already returns the allocated index.

Cc: stable@vger.kernel.org
Fixes: 53db8a71ecb4 ("io_uring: add support for IORING_OP_PIPE")
Signed-off-by: Jens Axboe <axboe@kernel.dk>