chore: fix race condition in watcher implementations

[neelayu]

There seems to be a race condition in fw.size when I ran the test.
I have used atomic.XXInt64 operations to ensure atomic read and writes.

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^TestWatchNotify$ github.com/influxdata/tail/watch -race -v logtostderr

=== RUN   TestWatchNotify
=== RUN   TestWatchNotify/Test_watch_inotify
Modified
==================
WARNING: DATA RACE
Write at 0x00c000012028 by goroutine 9:
  github.com/influxdata/tail/watch.(*InotifyFileWatcher).ChangeEvents()
      /Users/neelay.upadhyaya/workspace/tail/watch/inotify.go:75 +0x22a
  github.com/influxdata/tail/watch.watchFile()
      /Users/neelay.upadhyaya/workspace/tail/watch/watch_test.go:136 +0x214
  github.com/influxdata/tail/watch.TestWatchNotify.func1.1()
      /Users/neelay.upadhyaya/workspace/tail/watch/watch_test.go:49 +0xa4

Previous read at 0x00c000012028 by goroutine 12:
  github.com/influxdata/tail/watch.(*InotifyFileWatcher).ChangeEvents.func1()
      /Users/neelay.upadhyaya/workspace/tail/watch/inotify.go:135 +0x529

Logs are truncated, but they are also present in other places.

[neelayu]

@powersj can you check out this PR? Thanks!

[powersj]

Thanks - are you planning on making additional changes in a future PR?

[neelayu]

Good guess indeed!. I do have a change planned. Hopefully its okay to contribute 😅
I would be adding a watcher event support for file create as well, when the watcher is on a directory. Do you see this as a fruitful change? I intend to use it in telegraf

[powersj]

Do you see this as a fruitful change?

+1 on fixing tests, and we can always review your PR, could you file an issue to layout what you are doing?

To be honest, I don't think this project has had much attention, it doesn't look like CI is even set up anymore, so I can look into that as well.

[neelayu]

Thanks. I have created an issue. You can take a look at it.

[srebhan]

Looks good to me. Thanks for this fix @neelayu!