xfs: online repair of rmap/quota/summary counters

In this sixth part of the twentieth revision of the online repair
patchset, we provide all the repair features that require the entire
filesystem to be frozen.

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

Patch 2 is unnecessary?  It provides async io for xfile, which has
massive overhead costs but mostly shuts up lockdep.

Patch 3 implements reverse mapping btree reconstruction.

Patch 4 reimplements quotacheck as an online operation.

Patch 5 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.