ofp-util: Rename OpenFlow 1.1 buckets decoder.

Rename ofputil_pull_buckets() as ofputil_pull_buckets11()
to denote that it decodes OpenFlow 1.1 buckets (also used in
OpenFlow 1.2 to 1.4).

This refactoring is in preparation for supporting decoding
of (draft) OpenFlow 1.5 group mod messages and their
buckets.

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