io_uring: make io_uring_sqe_cmd() unconditionally available

If CONFIG_IO_URING isn't set, then io_uring_sqe_cmd() is not defined.
As the nvme driver uses this helper, it causes a compilation issue:

 drivers/nvme/host/ioctl.c: In function 'nvme_uring_cmd_io':
 drivers/nvme/host/ioctl.c:555:44: error: implicit declaration of function 'io_uring_sqe_cmd'; did you mean 'io_uring_free'? [-Werror=implicit-function-declaration]
   555 |         const struct nvme_uring_cmd *cmd = io_uring_sqe_cmd(ioucmd->sqe);
       |                                            ^~~~~~~~~~~~~~~~
       |                                            io_uring_free

Fix it by just making io_uring_sqe_cmd() generally available - the types
are known, and there's no reason to hide it under CONFIG_IO_URING.

Fixes: fd9b8547

 ("io_uring: Pass whole sqe to commands")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>