btrfs: add struct io_btrfs_cmd as type for io_uring_cmd_to_pdu()

Add struct io_btrfs_cmd as a wrapper type for io_uring_cmd_to_pdu(),
rather than using a raw pointer.

Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>