drm/ttm: fix use-after-free races in vm fault handling

commit 3089c1df10e2931b1d72d2ffa7d86431084c86b3 upstream.

The vm fault handler relies on the fact that the VMA owns a reference
to the BO. However, once mmap_sem is released, other tasks are free to
destroy the VMA, which can lead to the BO being freed. Fix two code
paths where that can happen, both related to vm fault retries.

Found via a lock debugging warning which flagged &bo->wu_mutex as
locked while being destroyed.

Fixes: cbe12e74 ("drm/ttm: Allow vm fault retries")
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>