xfs: directory parent pointers

Hi everyone,

Christoph and I have been working to get the directory parent pointers
patchset into shape for merging in the next kernel cycle.  This v13
release contains what I hope are the last ondisk format changes -- we've
gone back to parent pointers being xattrs attached to the child, wherein
the attr name is the dirent name, and the attr value is a handle to the
parent directory.

We've solved the pptr lookup uniqueness problem by forcing all
XFS_ATTR_PARENT attr lookups to be done on the name and value; avoided
namehash collisions on container farms by adjusting the hash function
slightly; and avoided the entire log incompat feature flag mess by
defining a permanent incompat feature for parent pointers and using
totally separate attr log item opcodes.

With that, I think this is finally ready to go.

Full versions are here:
https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xf...
https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xf...

--D