xfs: rename xfs_verify_rtext to xfs_verify_rtbext

This helper function validates that a range of *blocks* in the
realtime section is completely contained within the realtime section.
It does /not/ validate ranges of *rtextents*.  Rename the function to
avoid suggesting that it does, and change the type of the @len parameter
since xfs_rtblock_t is a position unit, not a length unit.

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