xfs: refactor rtbitmap/summary accessors [v1.2]

Since the rtbitmap and rtsummary accessor functions have proven more
controversial than the rest of the macro refactoring, split the patchset
into two to make review easier.

v1.1: various cleanups suggested by hch
v1.2: rework the accessor functions to reduce the amount of cursor
      tracking required, and create explicit bitmap/summary logging
      functions

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

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