io_uring/cmd: rename struct uring_cache to io_uring_cmd_data

In preparation for making this more generically available for
->uring_cmd() usage that needs stable command data, rename it and move
it to io_uring/cmd.h instead.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: David Sterba <dsterba@suse.com>

io_uring/uring_cmd.h

@@ -1,9 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
 
-struct uring_cache {
-	struct io_uring_sqe sqes[2];
-};
-
 int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags);
 int io_uring_cmd_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);