net/smc: Use reserve space when adding struct netns_smc in struct net

sangfor inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I76JHC



--------------------------------

When adding struct netns_smc to struct net, the size of struct net could
change, so use KABI_USE to fix the problem.

Signed-off-by: Litao Jiao <jiaolitao@sangfor.com.cn>