xfs: stabilize fs summary counters for online fsck

xfs: stabilize fs summary counters for online fsck

If the fscounters scrubber notices incorrect summary counters, it's
entirely possible that scrub is simply racing with other threads that
are updating the incore counters.  There isn't a good way to stabilize
percpu counters or set ourselves up to observe live updates with hooks
like we do for the quotacheck or nlinks scanners, so we instead choose
to freeze the filesystem long enough to walk the incore per-AG
structures.

Past me thought that it was going to be commonplace to have to freeze
the filesystem to perform some kind of repair and set up a whole
separate infrastructure to freeze the filesystem in such a way that
userspace could not unfreeze while we were running.  This involved
adding a mutex and freeze_super/thaw_super functions and dealing with
the fact that the VFS freeze/thaw functions can free the VFS superblock
references on return.

This was all very overwrought, since fscounters turned out to be the
only user of scrub freezes, and it doesn't require the log to quiesce,
only the incore superblock counters.  We prevent other threads from
changing the freeze level by calling freeze_super_excl with a custom
freeze cookie to keep everyone else out of the filesystem.

The end result is that fscounters should be much more efficient.  When
we're checking a busy system and we can't stabilize the counters, the
custom freeze will do less work, which should result in less downtime.
Repair should be similarly speedy, but that's in a later patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>