Fix too many dentry cache slab objs#7508.

[kaka11chen]

Fix #7508.

[drcrallen]

size must actually check the object, and depending on the collection, may have to traverse or do other special record keeping operations. Some collections can optimize this and others cannot. isEmpty() should be used when simply checking for emptyness.

[kaka11chen]

Good catch. Thanks.

[drcrallen]

There's another hidden FS bug here:

synchronized (files) {
  if (closed) {
    return;
  }
  ...    

If two methods call this, and the first one that finishes causes the JVM to be shut down, then the other one may never finish and actually do the delete. It is at the author's discretion if they want to keep the scope of this PR to just #7508 or to fix up `LimitedTemporaryStorage::close`` in general

If the author wishes to keep the scope limited, then I'm 👍 once the isEmpty change is made

[kaka11chen]

@drcrallen I have changed the source code with checking a flag createdStorageDirectory.