tipc: Refactor __tipc_nl_compat_doit

As preparation for adding RTNL to make (*cmd->transcode)() and
(*cmd->transcode)() constantly protected by RTNL lock, we move out of
memory allocations existing between them as many as possible so that
the time of holding RTNL can be minimized in __tipc_nl_compat_doit().

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>