Various minore fixes in the amdgpu plugin

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (8b5f3af) 70.52% compared to head (2739b2a) 70.51%. Report is 1 commits behind head on criu-dev.

:exclamation: Current head 2739b2a differs from pull request most recent head 8f92bd5. Consider uploading reports for the commit 8f92bd5 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## criu-dev #2274 +/- ## ============================================ - Coverage 70.52% 70.51% -0.02% ============================================ Files 132 132 Lines 33508 33508 ============================================ - Hits 23632 23628 -4 - Misses 9876 9880 +4 ``` | [Files](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2274?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore) | Coverage Δ | | |---|---|---| | [criu/files-reg.c](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2274?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore#diff-Y3JpdS9maWxlcy1yZWcuYw==) | `75.38% <0.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/checkpoint-restore/criu/pull/2274/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=checkpoint-restore)

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[rst0git]

I tested these changes locally with AMD MI100, and I can confirm that the checkpoint/restore works as expected.