xfs: online repair support (again)

This is the second part of the eighteenth revision of a patchset that
adds to XFS kernel support for online metadata scrubbing and repair.
There aren't any on-disk format changes.

New for this version is a rebase against 4.20 and the creation of
explicit data structures for storing intermediate reconstructed records.
Otherwise, there hasn't been much activity here, as we've been too busy
fixing clone/copy/deduperange ioctls.

Patch 1 gives scrub the ability to freeze the filesystem so that it can
do scans and repairs in mostly-exclusive mode.

Patch 2 implements reverse mapping btree reconstruction.

Patch 3 reimplements quotacheck as an online operation.

Patch 4 implements summary counter check and repair.