implement linkat

[izabera]

this implements .tmpfile and .link
it also abstracts the common path of flushing and linking into a helper function

[bitonic]

Nice, we've been meaning to add this for ages. Before I review, I'd change the main tests so that they take an option to create a file with tmpfile + link, and then run them with that option and check that things are fine. And I'd also just add a very basic test that checks if that works.

[mcrnic]

You have some bugs around dget_parent dput

Issues:

1. you call dget_parent in ternfs_link but it will not be put if flush_and_link errors
2. you don't call dget_parent in ternfs_file_flush but then you do dput in flush_and_link if it does not errors

[izabera]

made it always dget/dput the parent

[izabera]

ah i think i can scope it better