xfs/185: update for rtgroups

xfs/185: update for rtgroups

This old test is a bit too fixated on exact rt allocator behavior.  With
rtgroups enabled, we can end up with one large contiguous region that's
split into multiple bmbt mappings to avoid crossing rtgroup boundaries.
The realtime superblock throws another twist into the mix because the
first rtx will always be in use, which can shift the start of the
physical space mappings by up to 1 rtx.

Also fix a bug where we'd try to fallocate the total number of rtx,
whereas we should be asking for the number of free rtx to avoid ENOSPC
errors.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>