While QAing the new xfs_repair quotacheck code, I uncovered a quota
corruption bug resulting from a bad interaction between dquot buffer
initialization and quotacheck.  The bug can be reproduced with the
following sequence:

# mkfs.xfs -f /dev/sdf
# mount /dev/sdf /opt -o usrquota
# su nobody -s /bin/bash -c 'touch /opt/barf'
# sync
# xfs_quota -x -c 'report -ahi' /opt
User quota on /opt (/dev/sdf)
                        Inodes
User ID      Used   Soft   Hard Warn/Grace
---------- ---------------------------------
root            3      0      0  00 [------]
nobody          1      0      0  00 [------]

# xfs_io -x -c 'shutdown' /opt
# umount /opt
# mount /dev/sdf /opt -o usrquota
# touch /opt/man2
# xfs_quota -x -c 'report -ahi' /opt
User quota on /opt (/dev/sdf)
                        Inodes
User ID      Used   Soft   Hard Warn/Grace
---------- ---------------------------------
root            1      0      0  00 [------]
nobody          1      0      0  00 [------]

# umount /opt

Notice how the initial quotacheck set the root dquot icount to 3
(rootino, rbmino, rsumino), but after shutdown -> remount -> recovery,
xfs_quota reports that the root dquot has only 1 icount.  We haven't
deleted anything from the filesystem, which means that quota is now
under-counting.  This behavior is not limited to icount or the root
dquot, but this is the shortest reproducer.

I traced the cause of this discrepancy to the way that we handle ondisk
dquot updates during quotacheck vs. regular fs activity.  Normally, when
we allocate a disk block for a dquot, we log the buffer as a regular
(dquot) buffer.  Subsequent updates to the dquots backed by that block
are done via separate dquot log item updates, which means that they
depend on the logged buffer update being written to disk before the
dquot items.  Because individual dquots have their own LSN fields, that
initial dquot buffer must always be recovered.

However, the story changes for quotacheck, which can cause dquot block
allocations but persists the final dquot counter values via a delwri
list.  Because recovery doesn't gate dquot buffer replay on an LSN, this
means that the initial dquot buffer can be replayed over the (newer)
contents that were delwritten at the end of quotacheck.  In effect, this
re-initializes the dquot counters after they've been updated.  If the
log does not contain any other dquot items to recover, the obsolete
dquot contents will not be corrected by log recovery.

Because quotacheck uses a transaction to log the setting of the CHKD
flags in the superblock, we skip quotacheck during the second mount
call, which allows the incorrect icount to remain.

Fix this by changing the ondisk dquot initialization function to use
ordered buffers to write out fresh dquot blocks if it detects that we're
running quotacheck.  If the system goes down before quotacheck can
complete, the CHKD flags will not be set in the superblock and the next
mount will run quotacheck again, which can fix uninitialized dquot
buffers.  This requires amending the defer code to maintaine ordered
buffer state across defer rolls for the sake of the dquot allocation
code.

For regular operations we preserve the current behavior since the dquot
items require properly initialized ondisk dquot records.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>