btrfs/ioctl: store btrfs_uring_encoded_data in io_btrfs_cmd

btrfs/ioctl: store btrfs_uring_encoded_data in io_btrfs_cmd

btrfs is the only user of struct io_uring_cmd_data and its op_data
field. Switch its ->uring_cmd() implementations to store the struct
btrfs_uring_encoded_data * in the struct io_btrfs_cmd, overlayed with
io_uring_cmd's pdu field. This avoids having to touch another cache line
to access the struct btrfs_uring_encoded_data *, and allows op_data and
struct io_uring_cmd_data to be removed.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/20250708202212.2851548-4-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>