xfs: clean up realtime type usage [v1.1]

The realtime code uses xfs_rtblock_t and xfs_fsblock_t in a lot of
places, and it's very confusing.  Clean up all the type usage so that an
xfs_rtblock_t is always a block within the realtime volume, an
xfs_fileoff_t is always a file offset within a realtime metadata file,
and an xfs_rtxnumber_t is always a rt extent within the realtime volume.

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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