openflow: Use *_array_len names in struct ofp15_bucket and ofp15_group_mod

The spec has been clarified to use _list_len in palce of _list_len
terminology to make it clearer that the data is not an ordered list
(it is a set). The code present in Open vSwitch already avoided
the _list_len terminology. This change brings the code into
line with the updated spec.

ONF-JIRA: EXT-350
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>