xfs: minor bugfixes for rt stuff [v1.1]

This is a preparatory patchset that fixes a few miscellaneous bugs
before we start in on larger cleanups of realtime units usage.

v1.1: various cleanups suggested by hch

With a bit of luck, this should all go splendidly.

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