xfs: online repair of extended attributes

This series employs atomic extent swapping to enable safe reconstruction
of extended attribute data.  Extended attribute repair consists of four
main parts:

First, we walk the existing attributes to salvage as many of them as we
can, by adding them as new attributes attached to the repair tempfile.

Second, we prepare the temp file by changing the inode owner field in
the attr block headers.

Third, we use atomic extent swaps to exchange the entire attribute fork
between the two files.

Finally, we tear down the old attr data (which is now in the temporary
file) as carefully as we can.
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