kvclient panic when kill pd leader during br backup

[Lily2025]

1、run br backup
2、kill pd leader
3、br backup failed
`panic: close of closed channel

goroutine 2822 [running]:
github.com/tikv/client-go/v2/internal/client.(*batchConn).Close(...)
/root/go/pkg/mod/github.com/tikv/client-go/[email protected]/internal/client/client_batch.go:769
github.com/tikv/client-go/v2/internal/client.(*connArray).Close(0xc0004bb560)
/root/go/pkg/mod/github.com/tikv/client-go/[email protected]/internal/client/client.go:330 +0xf6
github.com/tikv/client-go/v2/internal/client.(*RPCClient).CloseAddr(0xc0007e3680, {0xc000f7e7d0, 0x44})
/root/go/pkg/mod/github.com/tikv/client-go/[email protected]/internal/client/client.go:797 +0x1e5
github.com/tikv/client-go/v2/internal/client.(*RPCClient).recycleIdleConnArray(0xc0007e3680)
/root/go/pkg/mod/github.com/tikv/client-go/[email protected]/internal/client/client_batch.go:835 +0x1f7
created by github.com/tikv/client-go/v2/internal/client.(*RPCClient).sendRequest in goroutine 2821
/root/go/pkg/mod/github.com/tikv/client-go/[email protected]/internal/client/client.go:587 +0x34b`

tidb version:
./tidb-server -V
Release Version: v7.5.2
Edition: Community
Git Commit Hash: 39ea2b30d32ab8cf486f30ca318cf2e4bd99eaef
Git Branch: HEAD
UTC Build Time: 2024-05-29 15:07:12
GoVersion: go1.21.10
Race Enabled: false
Check Table Before Drop: false
Store: unistore
2024-05-30T14:14:32.686+0800

[crazycs520]

fixed by #1359