xfs: remove the separate rtgroup and rtsb flags

xfs: remove the separate rtgroup and rtsb flags

Merge the rtsb and rtgroup flags into metadir.  A metadir file system now
always needs to use rtgroups if it has RT extents, and always has a super
block.  There still are separate feature checks to make the code more
obvious, and in case of rtsb to deal with the fact that the superblock
obviously only exists when there is a RT section (and the future zoned
RT device can't have one).

Also use the chance to move the metadir flag from the experimental to
the normal feature bit range.

Signed-off-by: Christoph Hellwig <hch@lst.de>