Add read-only FS_IOC_FSGETXATTR support

[a-shannon]

Summary

Add a read-only ioctl_fsgetxattr wrapper using the generated Linux
struct fsxattr definition.

This change:

• introduces a non-exhaustive Fsxattr result and named FsxattrFlags;
• returns all five non-padding fields from FS_IOC_FSGETXATTR;
• preserves unnamed returned FS_XFLAG_* bits through normal flag operations;
• zero-initializes the complete kernel object, including reserved padding;
• derives the ioctl opcode from the generated ABI type;
• adds no setter; and
• adds the missing CASEFOLD, ENCRYPTED, and VERITY names to IFlags.

Testing

Focused local validation recorded for both the linux_raw and use-libc
configurations:

• four integration tests covering flag handling, ABI layout, wrapper-versus-raw
  result parity, and unsupported-descriptor errno parity;
• the field-conversion and unknown-bit operator unit test;
• the generated-constant opcode check; and
• focused Clippy with warnings denied.

The broader local Clippy run remained blocked only by unchanged diagnostics in
timespec.rs. This does not claim repository-wide Clippy, MSRV, CI, or
cross-architecture runtime validation.

Related work

This is distinct from #1624, which adds FICLONERANGE. Both changes append
tests to tests/fs/ioctl.rs, so whichever lands second will need a small
test-file rebase.