io_uring/uring_cmd: get rid of using req->imu

It's pretty pointless to use io_kiocb as intermediate storage for this,
so split the validity check and the actual usage. The resource node is
assigned upfront at prep time, to prevent it from going away. The actual
import is never called with the ctx->uring_lock held, so grab it for
the import.

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