ovs-ofctl: Handle any number of buckets in group statistics

ovs-ofctl: Handle any number of buckets in group statistics

struct ofputil_group_stats has an arbitrary limit
of 16 buckets for which it can record statistics.
However the code does not appear to enforce this
limit and it seems to me that the code could overflow.

This patch aims to remove the arbitrary limit by
changing the 'bucket_stats' field of struct ofputil_group_stats
from a fixed length array to a pointer whose storage is allocated and freed
as necessary.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>