Revert "bcache: fix fifo index swapping condition in journal_pin_cmp()"

Coly says:

"Guoju Fang talked to me today, he told me this change was unnecessary
and I was over-thought.

Then I realize fifo_idx() uses a mask to handle the array index overflow
condition, so the index swap in journal_pin_cmp() won't happen. And yes,
Guoju and Kent are correct.

Since you already applied this patch, can you please to remove this
patch from your for-next branch? This single patch does not break
thing, but it is unecessary at this moment."

This reverts commit c0e0954e.

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