xfs: online repair of quota and rt metadata files

XFS stores quota records and free space bitmap information in files.
Add the necessary infrastructure to enable repairing metadata inodes and
their forks, and then make it so that we can repair the file metadata
for the rtbitmap.  Repairing the bitmap contents (and the summary file)
is left for subsequent patchsets.

We also add the ability to repair file metadata the quota files.  As
part of these repairs, we also reinitialize the ondisk dquot records as
necessary to get the incore dquots working.  We can also correct
obviously bad dquot record attributes, but we leave checking the
resource usage counts for the next patchsets.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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