ipv6: Fix signed integer overflow in __ip6_append_data

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

ipv6: Fix signed integer overflow in __ip6_append_data

Resurrect ubsan overflow checks and ubsan report this warning,
fix it by change the variable [length] type to size_t.

UBSAN: signed-integer-overflow in net/ipv6/ip6_output.c:1489:19
2147479552 + 8567 cannot be represented in type 'int'
CPU: 0 PID: 253 Comm: err Not tainted 5.16.0+ #1
Hardware name: linux,dummy-virt (DT)
Call trace:
dump_backtrace+0x214/0x230
show_stack+0x30/0x78
dump_stack_lvl+0xf8/0x118
dump_stack+0x18/0x30
ubsan_epilogue+0x18/0x60
handle_overflow+0xd0/0xf0
__ubsan_handle_add_overflow+0x34/0x44
__ip6_append_data.isra.48+0x1598/0x1688
ip6_append_data+0x128/0x260
udpv6_sendmsg+0x680/0xdd0
inet6_sendmsg+0x54/0x90
sock_sendmsg+0x70/0x88
____sys_sendmsg+0xe8/0x368
___sys_sendmsg+0x98/0xe0
__sys_sendmmsg+0xf4/0x3b8
__arm64_sys_sendmmsg+0x34/0x48
invoke_syscall+0x64/0x160
el0_svc_common.constprop.4+0x124/0x300
do_el0_svc+0x44/0xc8
el0_svc+0x3c/0x1e8
el0t_64_sync_handler+0x88/0xb0
el0t_64_sync+0x16c/0x170

Changes since v1:
-Change the variable [length] type to unsigned, as Eric Dumazet suggested.
Changes since v2:
-Don't change exthdrlen type in ip6_make_skb, as Paolo Abeni suggested.
Changes since v3:
-Don't change ulen type in udpv6_sendmsg and l2tp_ip6_sendmsg, as
Jakub Kicinski suggested.