Btrfs: add sanity tests for new qgroup accounting code

This exercises the various parts of the new qgroup accounting code.  We do some
basic stuff and do some things with the shared refs to make sure all that code
works.  I had to add a bunch of infrastructure because I needed to be able to
insert items into a fake tree without having to do all the hard work myself,
hopefully this will be usefull in the future.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index 10db21f..f09aa18 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -900,7 +900,11 @@
 		goto out;
 	BUG_ON(ret == 0);
 
+#ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
+	if (trans && likely(trans->type != __TRANS_DUMMY)) {
+#else
 	if (trans) {
+#endif
 		/*
 		 * look if there are updates for this ref queued and lock the
 		 * head