Unnecessary pessimistic rollback is sent on ErrKeyExists

[sticnarf]

Bug Report

1. Minimal reproduce step (Required)

It is a regression caused by #22976.

#22976 created a ErrKeyExists wrapper in store/tikv. Then, the type match at https://github.com/sticnarf/tidb/blob/375de3758c/store/tikv/txn.go#L472 was incorrect. Unnecessary pessimistic rollback may be sent if we receive ErrKeyExists.

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[sticnarf]

#23686 has fixed it on master.

[ti-srebot]

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