external: normalize file id lookup and warn on fallback reopen

[cheese-cakee]

Why this PR is necessary

External regular-file restore can behave inconsistently when the same file[mnt_id:inode] is written in different numeric forms (for example decimal in one place and 0x form in another). In that case, mapping lookup may miss a logically identical external file id and restoration falls back unexpectedly.

This PR makes matching robust for equivalent numeric ids and improves diagnostics when fallback is used.

What this PR changes

• Normalize parsing of file[mnt_id:inode] during external id lookup so decimal and 0x forms are treated as equal by value.
• Preserve exact string-match as fast path.
• Add a warning when no --inherit-fd mapping is found for an external file id and restore falls back to reopen-by-path.
• Clarify CLI/docs text that mnt_id and inode in file[mnt_id:inode] may be decimal or 0x hex.

Scope

• Single logical change.
• No unrelated modifications.

Testing

• git diff --check HEAD~1..HEAD
• python3 -m py_compile test/zdtm.py test/zdtm/criu_config.py

[Copilot]

Pull request overview

This PR aims to make external regular-file id matching more robust across equivalent numeric representations (decimal vs hex) and improve restore diagnostics when CRIU falls back from --inherit-fd to path-based reopen.

Changes:

• Add normalization logic for file[mnt_id:inode] matching in external id lookup (keeping exact string compare as a fast path).
• Emit a warning during restore when an external file has no --inherit-fd mapping and CRIU falls back to reopening by path.
• Update CLI help and documentation to describe accepted numeric formats for file[mnt_id:inode].

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
criu/files-reg.c	Adds a warning when --inherit-fd mapping is missing for an external file.
criu/external.c	Introduces parsing/normalization for file[mnt_id:inode] during external id lookup.
criu/crtools.c	Updates --external / --inherit-fd help text for file[mnt_id:inode] formats.
Documentation/criu.txt	Clarifies documentation for --external file[mnt_id:inode] numeric formats.