[starnix][overlayfs] Delegate file operation directly to ops

Instead of delegating to the FileObject, delegate to the ops.
This short circuit multiple locks and permissions check that should
already have happened on the higher level.

Change-Id: Ifa04f658615541579d6b58cc39a3bb3edbf23962
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1534557
Reviewed-by: Wez <wez@google.com>
Fuchsia-Auto-Submit: Benjamin Lerman <qsr@google.com>
Commit-Queue: Benjamin Lerman <qsr@google.com>