Btrfs: add a sanity test for btrfs_split_item

While looking at somebodys corruption I became completely convinced that
btrfs_split_item was broken, so I wrote this test to verify that it was working
as it was supposed to.  Thankfully it appears to be working as intended, so just
add this test to make sure nobody breaks it in the future.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index fa117f7d..ee0b8aa 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1724,7 +1724,9 @@
 	int ref_cows;
 	int track_dirty;
 	int in_radix;
-
+#ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
+	int dummy_root;
+#endif
 	u64 defrag_trans_start;
 	struct btrfs_key defrag_progress;
 	struct btrfs_key defrag_max;