dma-buf: Fix NULL pointer dereference in sanitycheck()
In the Linux kernel, the following vulnerability has been resolved:

dma-buf: Fix NULL pointer dereference in sanitycheck()

If due to a memory allocation failure mock_chain() returns NULL, it is
passed to dma_fence_enable_sw_signaling() resulting in NULL pointer
dereference there.

Call dma_fence_enable_sw_signaling() only if mock_chain() succeeds.

Found by Linux Verification Center (linuxtesting.org) with SVACE.