bufferhubd: Fix an error on not closing post_fence explicitly.

There is a typo in the code that instead of calling close() on a Post
fence, actually did get_fd() which is really a no-op.

Fixed by changing the call to call() explicitly.

Bug: None
Test: `m -j32` succeeds. Sailfish boots, CubeSea works.
Change-Id: If788aa8ea291317f0618724ecc35a4b355575772