drm/amdgpu: avoid double drm_exec_fini() in userq validate

When new_addition is true, amdgpu_userq_vm_validate() calls
drm_exec_fini(&exec) before iterating over the collected HMM ranges and
calling amdgpu_ttm_tt_get_user_pages().

If amdgpu_ttm_tt_get_user_pages() fails in that path, the code jumps to
unlock_all and calls drm_exec_fini(&exec) a second time on the same
exec object. drm_exec_fini() is not idempotent: it frees exec->objects
and may also drop exec->contended and finalize the ww acquire context.

Route that error path directly to the range cleanup once exec has
already been finalized.

Fixes: 42f148788469 ("drm/amdgpu/userqueue: validate userptrs for userqueues")
Issue found using a prototype static analysis tool
and confirmed by code review.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Hongyan Xu <getshell@seu.edu.cn>
Signed-off-by: Slavin Liu <220245772@seu.edu.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 2802952e4a07306da6ebe813ff1acacc5691851a)