[XFS] get_bulkall() could return incorrect inode state

In the following scenario xfs_bulkstat() returns incorrect stale inode
state:

1. File_A is created and its inode synced to disk. 2. File_A is unlinked
and doesn't exist anymore. 3. Filesystem sync is invoked. 4. File_B is
created. File_B happens to reclaim File_A's inode. 5. xfs_bulkstat() is
called and detects File_B but reports the

incorrect File_A inode state.

Explanation for the incorrect inode state is that inodes are not
immediately synced on file create for performance reasons. This leaves the
on-disk inode buffer uninitialized (or with old state from a previous
generation inode) and this is what xfs_bulkstat() would report.

The patch marks the on-disk inode buffer "dirty" on unlink. When the inode
is reclaimed (by a new file create), xfs_bulkstat() would filter this
inode by the "dirty" mark. Once the inode is flushed to disk, the on-disk
buffer "dirty" mark is automatically removed and a following
xfs_bulkstat() would return the correct inode state.

Marking the on-disk inode buffer "dirty" on unlink is achieved by setting
the on-disk di_nlink field to 0. Note that the in-core di_nlink has
already been set to 0 and a corresponding transaction logged by
xfs_droplink(). This is an exception from the rule that any on-disk inode
buffer changes has to be followed by a disk write (inode flush).
Synchronizing the in-core to on-disk di_nlink values in advance (before
the actual inode flush to disk) should be fine in this case because the
inode is already unlinked and it would never change its di_nlink again for
this inode generation.

SGI-PV: 970842
SGI-Modid: xfs-linux-melb:xfs-kern:29757a

Signed-off-by: Vlad Apostolov <vapo@sgi.com>
Signed-off-by: Alex Elder <aelder@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Mark Goodwin <markgw@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>