libfrog: create a new scrub group for things requiring full inode scans

libfrog: create a new scrub group for things requiring full inode scans

Subsequent patches will add online fsck types (quotacheck, link counts)
that require us to walk every inode in the entire filesystem.  This
requires the AG metadata and the inodes to be in good enough shape to
complete the scan without hitting corruption errors.  As such, they
ought to run after phases 2-4 and before phase 7, which summarizes what
we've found.

Phase 5 seems like a reasonable place to do this, since it already walks
every xattr and directory entry in the filesystem to look for suspicious
looking names.  Add a new XFROG_SCRUB_GROUP, and add it to phase 5.

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