Linux - Harden mountinfo API and several related fixes

[gcmoreira]

In the context of #1516 and related issues, this PR addresses several issues in the mountinfo API code and its associated functions.

This PR includes:

• Fix container_of(). Since object() can create and return an invalid object, we ensure this method will return a valid one checking it address using the layer.
• Ensure _get_tasks_mountpoints() will always return valid mountpoint checkin all the pointers involved.
• Ensure do_get_path() will always return a valid string.
• Fix struct_file by removing invalid members. The f_dentry and f_vfsmnt members were incorrectly included, though they were never part of struct file. These were preprocessor macros used as shortcuts within kernel code but should not be present in the structure itself.
• Docstring improvements

[ikelos]

Generally happy with all of this, although not sure why the function signature for one bit changed (and without that change, there's no need for version bumps that I can see). Needs a response to some of the points raised but no showstoppers at least...

[gcmoreira]

@ikelos thanks for the review. All the observations were addressed

[gcmoreira]

@ikelos hold on with this as it seems there's a commit missing

[gcmoreira]

@ikelos no worries, it's all good! Turns out it was just how GitHub was displaying the changes, pretty confusing at first

[ikelos]

Ok, looks good, let's go for it. 5:)