lib/ukfile: Remove padding from struct uk_statx

[andreittr]

Description of changes

This change removes the padding fields from uk_statx, minimizing memory waste when allocating inside the kernel.
Syscalls never reveal these kernel structs, and userspace allocates a struct statx dictated by its libc.

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.uk on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): [e.g. x86_64 or N/A]
• Platform(s): [e.g. kvm, xen or N/A]
• Application(s): [e.g. app-python3 or N/A]

Additional configuration

N/A