fix(utils): map common Linux filesystem magic values

[overtrue]

Summary

• Add tested Linux filesystem magic mappings for XFS and BTRFS.
• Move the filesystem magic mapping helper into a small shared module so the mapping can be unit-tested on non-Linux hosts.
• Preserve the unverified, less common magic values as TODOs instead of guessing mappings.

Backlog: rustfs/backlog#646

Validation

• RED: PATH="$HOME/.cargo/bin:$PATH" cargo test -p rustfs-utils --features os map_common_linux_filesystem_magic_numbers -- --nocapture failed with UNKNOWN for XFS before adding the mappings.
• GREEN: PATH="$HOME/.cargo/bin:$PATH" cargo test -p rustfs-utils --features os map_common_linux_filesystem_magic_numbers -- --nocapture
• PATH="$HOME/.cargo/bin:$PATH" cargo test -p rustfs-utils --features os os:: -- --nocapture
• PATH="$HOME/.cargo/bin:$PATH" cargo fmt --all --check
• PATH="$HOME/.cargo/bin:$PATH" cargo clippy -p rustfs-utils --features os --tests -- -D warnings

Performance

• The runtime work remains a single match on the filesystem magic number. The refactor only moves the helper for testability and adds two constant arms.

[github-actions]

CLA requirements are satisfied for this pull request.

[Copilot]

Pull request overview

This PR improves Linux filesystem type detection in rustfs-utils by adding verified magic-number mappings for XFS and BTRFS, and refactoring the mapping into a shared module so it can be unit-tested even on non-Linux hosts.

Changes:

• Added XFS (0x58465342) and BTRFS (0x9123683E) filesystem magic mappings.
• Moved the filesystem magic mapping helper out of os/linux.rs into a new os/fs_type.rs module.
• Added unit tests covering the new magic-number mappings and preserved unverified mappings as TODOs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/utils/src/os/mod.rs	Adds the new fs_type module to the OS utilities module tree.
crates/utils/src/os/linux.rs	Switches Linux disk info code to use the shared get_fs_type helper.
crates/utils/src/os/fs_type.rs	Introduces a shared filesystem-magic mapping helper and unit tests for XFS/BTRFS.