Btrfs: Correct redundant test in add_inode_ref

dir has already been tested.  It seems that this test should be on the
recently returned value inode.

A simplified version of the semantic match that finds this problem is as
follows: (http://www.emn.fr/x-info/coccinelle/)
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/tree-log.c

@@ -797,7 +797,7 @@ static noinline int add_inode_ref(struct btrfs_trans_handle *trans,
 		return -ENOENT;
 
 	inode = read_one_inode(root, key->objectid);
-	BUG_ON(!dir);
+	BUG_ON(!inode);
 
 	ref_ptr = btrfs_item_ptr_offset(eb, slot);
 	ref_end = ref_ptr + btrfs_item_size_nr(eb, slot);