xfs: create helpers to convert rt block numbers to rt extent numbers

xfs: create helpers to convert rt block numbers to rt extent numbers

Source kernel commit: 5dc3a80d46a450481df7f7e9fe673ba3eb4514c3

Create helpers to do unit conversions of rt block numbers to rt extent
numbers.  There are three variations -- one to compute the rt extent
number from an rt block number; one to compute the offset of an rt block
within an rt extent; and one to extract both.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>