btrfs: fix ref-verify to catch operations on 0 ref extents

While debugging I noticed I wasn't getting ref verify errors before
everything blew up.  Turns out it's because we don't warn when we try to
add a normal ref via btrfs_inc_ref() if the block entry exists but has 0
references.  This is incorrect, we should never be doing anything other
than adding a new extent once a block entry drops to 0 references.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ref-verify.c

@@ -803,6 +803,15 @@ int btrfs_ref_tree_mod(struct btrfs_fs_info *fs_info,
 			kfree(ref);
 			kfree(ra);
 			goto out_unlock;
+		} else if (be->num_refs == 0) {
+			btrfs_err(fs_info,
+		"trying to do action %d for a bytenr that has 0 total references",
+				action);
+			dump_block_entry(fs_info, be);
+			dump_ref_action(fs_info, ra);
+			kfree(ref);
+			kfree(ra);
+			goto out_unlock;
 		}
 
 		if (!parent) {