GFS2: Fix use-after-free race when calling gfs2_remove_from_ail

Function gfs2_remove_from_ail drops the reference on the bh via
brelse. This patch fixes a race condition whereby bh is deferenced
after the brelse when setting bd->bd_blkno = bh->b_blocknr;
Under certain rare circumstances, bh might be gone or reused,
and bd->bd_blkno is set to whatever that memory happens to be,
which is often 0. Later, in gfs2_trans_add_unrevoke, that bd fails
the test "bd->bd_blkno >= blkno" which causes it to never be freed.
The end result is that the bd is never freed from the bufdata cache,
which results in this error:
slab error in kmem_cache_destroy(): cache `gfs2_bufdata': Can't free all objects

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/log.c

@@ -551,10 +551,10 @@ void gfs2_add_revoke(struct gfs2_sbd *sdp, struct gfs2_bufdata *bd)
 	struct buffer_head *bh = bd->bd_bh;
 	struct gfs2_glock *gl = bd->bd_gl;
 
-	gfs2_remove_from_ail(bd);
-	bd->bd_bh = NULL;
 	bh->b_private = NULL;
 	bd->bd_blkno = bh->b_blocknr;
+	gfs2_remove_from_ail(bd); /* drops ref on bh */
+	bd->bd_bh = NULL;
 	bd->bd_ops = &gfs2_revoke_lops;
 	sdp->sd_log_num_revoke++;
 	atomic_inc(&gl->gl_revokes);