[MINOR] Simple CacheIndexImpl#count refactoring to improve readability

[nizhikov]

…dability

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[nizhikov]

https://tcbot2.sbt-ignite-dev.ru/pr.html?serverId=apache&suiteId=IgniteTests24Java8_RunAll&branchForTc=pull/11517/head&action=Latest