ipv6: Fix an uninit variable access bug in __ip6_make_skb()

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

ipv6: Fix an uninit variable access bug in __ip6_make_skb()

Syzbot reported a bug as following:

=====================================================
BUG: KMSAN: uninit-value in arch_atomic64_inc arch/x86/include/asm/atomic64_64.h:88 [inline]
BUG: KMSAN: uninit-value in arch_atomic_long_inc include/linux/atomic/atomic-long.h:161 [inline]
BUG: KMSAN: uninit-value in atomic_long_inc include/linux/atomic/atomic-instrumented.h:1429 [inline]
BUG: KMSAN: uninit-value in __ip6_make_skb+0x2f37/0x30f0 net/ipv6/ip6_output.c:1956
arch_atomic64_inc arch/x86/include/asm/atomic64_64.h:88 [inline]
arch_atomic_long_inc include/linux/atomic/atomic-long.h:161 [inline]
atomic_long_inc include/linux/atomic/atomic-instrumented.h:1429 [inline]
__ip6_make_skb+0x2f37/0x30f0 net/ipv6/ip6_output.c:1956
ip6_finish_skb include/net/ipv6.h:1122 [inline]
ip6_push_pending_frames+0x10e/0x550 net/ipv6/ip6_output.c:1987
rawv6_push_pending_frames+0xb12/0xb90 net/ipv6/raw.c:579
rawv6_sendmsg+0x297e/0x2e60 net/ipv6/raw.c:922
inet_sendmsg+0x101/0x180 net/ipv4/af_inet.c:827
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg net/socket.c:734 [inline]
____sys_sendmsg+0xa8e/0xe70 net/socket.c:2476
___sys_sendmsg+0x2a1/0x3f0 net/socket.c:2530
__sys_sendmsg net/socket.c:2559 [inline]
__do_sys_sendmsg net/socket.c:2568 [inline]
__se_sys_sendmsg net/socket.c:2566 [inline]
__x64_sys_sendmsg+0x367/0x540 net/socket.c:2566
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

Uninit was created at:
slab_post_alloc_hook mm/slab.h:766 [inline]
slab_alloc_node mm/slub.c:3452 [inline]
__kmem_cache_alloc_node+0x71f/0xce0 mm/slub.c:3491
__do_kmalloc_node mm/slab_common.c:967 [inline]
__kmalloc_node_track_caller+0x114/0x3b0 mm/slab_common.c:988
kmalloc_reserve net/core/skbuff.c:492 [inline]
__alloc_skb+0x3af/0x8f0 net/core/skbuff.c:565
alloc_skb include/linux/skbuff.h:1270 [inline]
__ip6_append_data+0x51c1/0x6bb0 net/ipv6/ip6_output.c:1684
ip6_append_data+0x411/0x580 net/ipv6/ip6_output.c:1854
rawv6_sendmsg+0x2882/0x2e60 net/ipv6/raw.c:915
inet_sendmsg+0x101/0x180 net/ipv4/af_inet.c:827
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg net/socket.c:734 [inline]
____sys_sendmsg+0xa8e/0xe70 net/socket.c:2476
___sys_sendmsg+0x2a1/0x3f0 net/socket.c:2530
__sys_sendmsg net/socket.c:2559 [inline]
__do_sys_sendmsg net/socket.c:2568 [inline]
__se_sys_sendmsg net/socket.c:2566 [inline]
__x64_sys_sendmsg+0x367/0x540 net/socket.c:2566
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

It is because icmp6hdr does not in skb linear region under the scenario
of SOCK_RAW socket. Access icmp6_hdr(skb)->icmp6_type directly will
trigger the uninit variable access bug.

Use a local variable icmp6_type to carry the correct value in different
scenarios.