fix(initrd): corrupted symlinks

[antoineco]

Prerequisite checklist

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

Description of changes

Due to an issue upstream (cavaliergopher/cpio#15) files with mode Symlink are written as zero-sized files, without symlink target:

Output of Unikraft:

Info: [libukcpio] <cpio.c @  298> Creating symlink /bin/arch
Info: [libukcpio] <cpio.c @  316> /bin/arch: Target is
dbg:  [libvfscore] <main.c @ 1668> (int) uk_syscall_r_symlink((const char*) 0x11eb0, (const char*) 0x43d2e0020)
ERR:  [libukcpio] <cpio.c @  318> /bin/arch: Failed to create symlink: Unknown error 0 (0)

Output of included tests:

=== RUN   TestNewFromMapping
    initrd_test.go:82: file [./lib/libtest.so.1]: got mode 0100000, expected 0120000
    initrd_test.go:85: file [./lib/libtest.so.1]: got linkname "", expected "libtest.so.1.0.0"
    initrd_test.go:88: file [./lib/libtest.so.1]: got size 16, expected 0

This PR refactors the NewFromMapping function to use a (hopefully temporary) fork which includes the relevant fix (cavaliergopher/cpio#16).

Additionally, I refactored the logic a little for readability.

[craciunoiuc]

Code looks good, will try on Monday 🙏

[antoineco]

I just learned @nderjung was already working on this.

[craciunoiuc]

it worked btw

[antoineco]

I know, I included tests.