net: fix uninit-value in caif_seqpkt_sendmsg

When nr_segs equal to zero in iovec_from_user, the object
msg->msg_iter.iov is uninit stack memory in caif_seqpkt_sendmsg
which is defined in ___sys_sendmsg. So we cann't just judge
msg->msg_iter.iov->base directlly. We can use nr_segs to judge
msg in caif_seqpkt_sendmsg whether has data buffers.

=====================================================
BUG: KMSAN: uninit-value in caif_seqpkt_sendmsg+0x693/0xf60 net/caif/caif_socket.c:542
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x220 lib/dump_stack.c:118
 kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
 __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
 caif_seqpkt_sendmsg+0x693/0xf60 net/caif/caif_socket.c:542
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 ____sys_sendmsg+0x12b6/0x1350 net/socket.c:2343
 ___sys_sendmsg net/socket.c:2397 [inline]
 __sys_sendmmsg+0x808/0xc90 net/socket.c:2480
 __compat_sys_sendmmsg net/compat.c:656 [inline]

Reported-by:  <syzbot+09a5d591c1f98cf5efcb@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?id=1ace85e8fc9b0d5a45c08c2656c3e91762daa9b8
Fixes: bece7b23

 ("caif: Rewritten socket implementation")
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>