net: fix uninit-value in caif_seqpkt_sendmsg

In the Linux kernel, the following vulnerability has been resolved:

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]