drm/amdgpu/sdma4: replace BUG_ON with WARN_ON in fence emission

drm/amdgpu/sdma4: replace BUG_ON with WARN_ON in fence emission

sdma_v4_0_ring_emit_fence() contains two BUG_ON(addr & 0x3) assertions
that verify fence writeback addresses are dword-aligned.  These
assertions can be reached from unprivileged userspace via crafted
DRM_IOCTL_AMDGPU_CS submissions, causing a fatal kernel panic in a
scheduler worker thread.

Replace both BUG_ON() calls with WARN_ON() to log the condition without
crashing the kernel.  A misaligned fence address at this point indicates
a driver bug, but crashing the kernel is never the correct response when
the assertion is reachable from userspace.

The CS IOCTL path is the correct place to filter invalid submissions;
the ring emission callback is too late to do anything about it.

Fixes: 2130f89ced2c ("drm/amdgpu: add SDMA v4.0 implementation (v2)")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: John B. Moore <jbmoore61@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit b90250bd933afd1ba94d86d6b13821997b22b18e)
Cc: stable@vger.kernel.org