dma-buf/dma-fence: fix checking signaling bit for timeline and driver name v3

In the Linux kernel, the following vulnerability has been resolved:

dma-buf/dma-fence: fix checking signaling bit for timeline and driver name v3

The patch "dma-buf: dma-fence: Fix potential NULL pointer dereference"
changed the check to test for the ops pointer instead of the signaled
bit to avoid a potential NULL dereference when the ops pointer has been
cleared.

The problem is now that the ops pointer is cleared only when neither the
release nor the wait callback is implemented and this isn't true for a lot
of dma_fence implementations yet. So those implementations lost the RCU
protection after signaling of the returned string resulting in potential
use after free.

Add the signaling check additional to the ops pointer check so that we
have both the protection against NULL dereference as well as the RCU
protection after signaling for the returned string.

v2: improve comments to note RCU protection and explain why we check
both signaling state and ops pointer
v3: some comment improvements suggested by Philip