HBASE-30340: CatalogJanitor can run concurrent scans due to incorrect alreadyRunning lock handling

[sercanCyberVision]

CatalogJanitor.scan() can allow concurrent scans due to incorrect handling of the
alreadyRunning lock.

ROOT CAUSE
Currently, the lock acquisition is performed inside the try block:

    try {
      if (!alreadyRunning.compareAndSet(false, true)) {
        return -1;
      }
      ...
    } finally {
      alreadyRunning.set(false);
    }

When a scan is already running, a concurrent scan fails the compareAndSet() and
returns immediately. However, because the lock acquisition is inside the try block,
the finally block is still executed and resets alreadyRunning to false.

This allows another scan to acquire the lock while the original scan is still running.

SOLUTION
The lock acquisition should be moved before the try block:

    if (!alreadyRunning.compareAndSet(false, true)) {
      return -1;
    }
    try {
      ...
    } finally {
      alreadyRunning.set(false);
    }

This prevents another scan from starting until the current scan has completed.