File metadata control

[l0kod]

We should be able to control access to file metadata (e.g. chmod, chgrp, setxattr, getxattr, utime). Some path-based LSM hooks enable to control a subset of these modifications, but we should provide users a safe and simple way to group these accesses, like with the following access rights:

• LANDLOCK_ACCESS_FS_READ_METADATA: read any file/dir metadata;
• LANDLOCK_ACCESS_FS_WRITE_SAFE_METADATA: change file times, user xattr;
• LANDLOCK_ACCESS_FS_WRITE_UNSAFE_METADATA: interpreted by the kernel, mostly xattr/chmod/chown that could change non-Landlock DAC or MAC, which could be considered as a policy bypass, or other various xattr that might be interpreted by filesystems. This should be denied most of the time.

This work depends on #9.

See https://lore.kernel.org/all/Ywpw66EYRDTQIyTx@nuc/