xfs: refactor rtbitmap/summary macros [v1.1]

In preparation for adding block headers and enforcing endian order in
rtbitmap and rtsummary blocks, replace open-coded geometry computations
and fugly macros with proper helper functions that can be typechecked.
Soon we'll be needing to add more complex logic to the helpers.

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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