xfs: online repair of directories

This series employs atomic extent swapping to enable safe reconstruction
of directory data.  Directory repair consists of five main parts:

First, we walk the existing directory to salvage as many entries as we
can, by adding them as new directory entries to the repair temp dir.

Second, we validate the parent pointer found in the directory.  If one
was not found, we scan the entire filesystem looking for a potential
parent.

Third, we prepare the temp file by changing the inode owner field in
the directory block headers.

Fourth, we use atomic extent swaps to exchange the entire data fork
between the two directories.

Finally, we add the ability to inactivate directories by directly
freeing all the data fork blocks.  This does not change anything with
normal directories, since they must still unlink and shrink one entry at
a time.