bcachefs: Only abort the transactions in the cycle

When the cycle doesn't involve the initiator of the cycle detection,
we might choose a transaction that is not involved in the cycle to abort.
It shouldn't be that since it won't break the cycle, this patch
therefore chooses the transaction in the cycle to abort.

Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>