drm/xe/sync: Fix user fence leak on alloc failure

drm/xe/sync: Fix user fence leak on alloc failure

When dma_fence_chain_alloc() fails, properly release the user fence
reference to prevent a memory leak.

Fixes: adda4e855ab6 ("drm/xe: Enforce correct user fence signaling order using")
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260219233516.2938172-6-shuicheng.lin@intel.com