repair: parallelise phase 7

repair: parallelise phase 7

It operates on a single AG at a time, sequentially, doing inode updates. All the
data structures accessed and modified are per AG, as are the modification to
on-disk structures. Hence we can run this phase concurrently across multiple
AGs.

This is important for large, broken filesystem repairs, where there can be
millions of inodes that need link counts updated. Once such repair image takes
more than 45 minutes to run phase 7 as a single threaded operation.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>