xfs: online repair support (pt. 2)

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 5.1, AG offlining, and a
reduction in the use of filesystem freezing to repair data structures.
Now we only use it to reset the summary counters if they were found to
be grossly incorrect.

The first two patches implement AG allocation offlining.  This teaches
the block and inode allocators to avoid marked AGs, which is the first
piece to being able to collect rmap repair information without freezing
the filesystem.

Patch 3 gives scrub the ability to freeze the filesystem so that it can
do scans and repairs in mostly-exclusive mode.

Patch 4 implements reverse mapping btree reconstruction.

Patch 5 reimplements quotacheck as an online operation.

Patch 6 implements a summary counter repair function.  In contrast to
its scrub-only counterpart, if we decide to repair the summary counters
we need to freeze the filesystem to prevent any concurrent operations
while we calculate the correct value for the summary counters.