btrfs: directly return 0 on no error code in btrfs_insert_raid_extent()

It's more obvious to return a literal zero instead of "return ret;".
Plus Smatch complains that ret could be uninitialized if the
ordered_extent->bioc_list list is empty and this silences that warning.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>