read file mode from RockRidge PX entry

[codecov[bot]]

Codecov Report

Patch coverage: 80.55% and project coverage change: +0.05% :tada:

Comparison is base (c7ce0b5) 78.66% compared to head (21ac86a) 78.71%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #44 +/- ## ========================================== + Coverage 78.66% 78.71% +0.05% ========================================== Files 6 6 Lines 1167 1203 +36 ========================================== + Hits 918 947 +29 - Misses 176 181 +5 - Partials 73 75 +2 ``` | [Files Changed](https://app.codecov.io/gh/kdomanski/iso9660/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski) | Coverage Δ | | |---|---|---| | [image\_writer.go](https://app.codecov.io/gh/kdomanski/iso9660/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-aW1hZ2Vfd3JpdGVyLmdv) | `75.95% <75.00%> (-0.03%)` | :arrow_down: | | [rockridge.go](https://app.codecov.io/gh/kdomanski/iso9660/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-cm9ja3JpZGdlLmdv) | `82.60% <80.00%> (-2.01%)` | :arrow_down: | | [image\_reader.go](https://app.codecov.io/gh/kdomanski/iso9660/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-aW1hZ2VfcmVhZGVyLmdv) | `77.16% <100.00%> (+0.57%)` | :arrow_up: |

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