mbuf_offload: fix header for C++

mbuf_offload: fix header for C++

When built in a C++ application, the include fails for 2 reasons:

rte_mbuf_offload.h:128:24: error:
invalid conversion from ‘void*’ to ‘rte_pktmbuf_offload_pool_private*’ [-fpermissive]
    rte_mempool_get_priv(mpool);
                        ^
The cast must be explicit for C++.

rte_mbuf_offload.h:304:1: error: expected declaration before ‘}’ token

There was a closing brace for __cplusplus but not an opening one.

Fixes: 78c8709b5ddb ("mbuf_offload: introduce library to attach offloads to mbuf")

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>