Btrfs: do not bug_on if we try to cow a free space cache inode

We can just return an error and we'll bail out properly.  We still want to catch
this case to make sure we don't have a bug somewhere, so just warn if this pops
up.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/inode.c

@@ -843,7 +843,10 @@ static noinline int cow_file_range(struct inode *inode,
 	struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
 	int ret = 0;
 
-	BUG_ON(btrfs_is_free_space_inode(inode));
+	if (btrfs_is_free_space_inode(inode)) {
+		WARN_ON_ONCE(1);
+		return -EINVAL;
+	}
 
 	num_bytes = ALIGN(end - start + 1, blocksize);
 	num_bytes = max(blocksize,  num_bytes);