xfs: convert xfs_extlen_t to xfs_rtxlen_t in the rt allocator

In most of the filesystem, we use xfs_extlen_t to store the length of a
file (or AG) space mapping in units of fs blocks.  Unfortunately, the
realtime allocator also uses it to store the length of a rt space
mapping in units of rt extents.  This is confusing, since one rt extent
can consist of many fs blocks.

Separate the two by introducing a new type (xfs_rtxlen_t) to store the
length of a space mapping (in units of realtime extents) that would be
found in a file.

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