xfs_scrub: call bulkstat directly if we're only scanning user files

xfs_scrub: call bulkstat directly if we're only scanning user files

Christoph observed xfs_scrub phase 5 consuming a lot of CPU time on a
filesystem with a very large number of rtgroups.  He traced this to
bulkstat_for_inumbers spending a lot of time trying to single-step
through inodes that were marked allocated in the inumbers record but
didn't show up in the bulkstat data.  These correspond to files in the
metadata directory tree that are not returned by the regular bulkstat.

This complex machinery isn't necessary for the inode walk that occur
during phase 5 because phase 5 wants to open user files and check the
dirent/xattr names associated with that file.  It's not needed for phase
6 because we're only using it to report data loss in unlinked files when
parent pointers aren't enabled.

Furthermore, we don't need to do this inumbers -> bulkstat dance because
phase 3 and 4 supposedly fixed any inode that was to corrupt to be
igettable and hence reported on by bulkstat.

Fix this by creating a simpler user file iterator that walks bulkstat
across the filesystem without using inumbers.  While we're at it, fix
the obviously incorrect comments in inodes.h.

Cc: <linux-xfs@vger.kernel.org> # v4.15.0
Fixes: 372d4ba99155b2 ("xfs_scrub: add inode iteration functions")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>