xfs: atomic file metadata repairs

This series connects the atomic extent swap mechanism in the previous
series to the online filesystem repair code.  This enables repair
functions to construct a clean copy of a directory, xattr information,
realtime bitmaps, and realtime summary information in a temporary inode.
If this completes successfully, the new contents can be swapped
atomically into the inode being repaired.  This is essential to avoid
making corruption problems worse if the system goes down in the middle
of running repair.