ofproto/ofproto-dpif: Use ofpbuf API to access 'data' and 'size'.

A recent commit reintroduced some direct use of the ofpbuf 'data' and
'size'.  Use the access API instead.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
lib/ofpbuf: Rename private fields to discourage direct use.

Direct use of 'data', 'base', and 'size' will break DPDK builds.  Try
to wean us off the habit by renaming the fields.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>