mrf: Avoid rare data race and more simplification

[vadmeste]

Description

This change avoids a rare data race and simplify the function that
returns MRF last activity information.

Motivation and Context

WARNING: DATA RACE
Write at 0x0000077a0cf8 by main goroutine:
  github.com/minio/minio/cmd.initHealMRF()
      github.com/minio/minio/cmd/mrf.go:233 +0x1e9
  github.com/minio/minio/cmd.serverMain()
      github.com/minio/minio/cmd/server-main.go:532 +0xe44
  github.com/minio/cli.HandleAction()
      github.com/minio/cli@v1.22.0/app.go:500 +0x103
  github.com/minio/cli.Command.Run()
      github.com/minio/cli@v1.22.0/command.go:225 +0xdc5
  github.com/minio/cli.(*App).Run()
      github.com/minio/cli@v1.22.0/app.go:261 +0xab3
  github.com/minio/minio/cmd.Main()
      github.com/minio/minio/cmd/main.go:168 +0xa8
  main.main()
      github.com/minio/minio/main.go:30 +0x64
Previous read at 0x0000077a0cf8 by goroutine 121:
  [failed to restore the stack]

How to test this PR?

Hard to catch the race warning, a normal MRF testing will be good

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Documentation updated
• Unit tests added/updated

[minio-trusted]

Mint Automation

Test	Result
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-gateway-s3.sh	✔️
mint-erasure.sh	✔️
mint-dist-erasure.sh	✔️
mint-zoned.sh	✔️
mint-gateway-nas.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
Deleting image on docker hub
Deleting image locally