libfrog: promote XFROG_SCRUB_DESCR_SUMMARY to a scrub type

libfrog: promote XFROG_SCRUB_DESCR_SUMMARY to a scrub type

"Summary" metadata, at least in the scrub context, are metadata whose
values depend on some kind of computation and therefore can only be
checked after we've looked at all the other metadata.  Currently, the
superblock summary counters are the only thing that are like this, but
since they run in a totally separate xfs_scrub phase (7 vs. 2), make
them their own group and remove the group+flag mix.

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