xfs: refactor rt extent unit conversions [v1.1]

This series replaces all the open-coded integer division and
multiplication conversions between rt blocks and rt extents with calls
to static inline helpers.  Having cleaned all that up, the helpers are
augmented to skip the expensive operations in favor of bit shifts and
masking if the rt extent size is a power of two.

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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