io_uring/uring_cmd: fix buffer index retrieval

Add back buffer index retrieval for IORING_URING_CMD_FIXED.

Reported-by: Guangwu Zhang <guazhang@redhat.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Fixes: b54a1404 ("io_uring/rsrc: add io_rsrc_node_lookup() helper")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Tested-by: Guangwu Zhang <guazhang@redhat.com>
Link: https://lore.kernel.org/r/20241111101318.1387557-1-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>