xfs: enable quota for realtime voluems

At some point, I realized that I've refactored enough of the quota code
in XFS that I should evaluate whether or not quota actually works on
realtime volumes.  It turns out that with two exceptions, it actually
does seem to work properly!  There are three broken pieces that I've
found so far: chown doesn't work, the quota accounting goes wrong when
the rt bitmap changes size, and the VFS quota ioctls don't report the
realtime warning counts or limits.  Hence this series fixes two things
in XFS and re-enables rt quota after a break of a couple decades.
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