xfs: cleanup xfs_getfsmap_rtdev_rtbitmap

Use mp->m_sb.sb_rblocks to calculate the end instead of sb_rextents that
needs a conversion, use consistent names to xfs_rtblock_t types, and
only calculated them by the time they are needed.  Remove the pointless
"high" local variable that only has a single user.

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