xfs_{db,repair}: convert open-coded xfs_rtword_t pointer accesses to helper

xfs_{db,repair}: convert open-coded xfs_rtword_t pointer accesses to helper

There are a bunch of places in xfs_db and xfs_repair where we use
open-coded logic to find a pointer to an xfs_rtword_t within a rt bitmap
buffer.  Convert all that to helper functions for better type safety.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>