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.