CVE-2021-47297 net: fix uninit-value in caif_seqpkt_sendmsg

In the Linux kernel, the following vulnerability has been resolved: net: fix uninit-value in caifseqpktsendmsg When nrsegs equal to zero in iovecfromuser, the object msg-msgiter.iov is uninit stack memory in caifseqpktsendmsg which is defined in syssendmsg. So we cann't just judge...