ipv6: Restore fib6_config validation for SIOCADDRT.

ipv6: Restore fib6_config validation for SIOCADDRT.

syzkaller reported out-of-bounds read in ipv6_addr_prefix(),
where the prefix length was over 128.

The cited commit accidentally removed some fib6_config
validation from the ioctl path.

Let's restore the validation.

[0]:
BUG: KASAN: slab-out-of-bounds in ip6_route_info_create (./include/net/ipv6.h:616 net/ipv6/route.c:3814)
Read of size 1 at addr ff11000138020ad4 by task repro/261

CPU: 3 UID: 0 PID: 261 Comm: repro Not tainted 6.15.0-rc3-00614-g0d15a26b247d #87 PREEMPT(voluntary)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl (lib/dump_stack.c:123)
 print_report (mm/kasan/report.c:409 mm/kasan/report.c:521)
 kasan_report (mm/kasan/report.c:636)
 ip6_route_info_create (./include/net/ipv6.h:616 net/ipv6/route.c:3814)
 ip6_route_add (net/ipv6/route.c:3902)
 ipv6_route_ioctl (net/ipv6/route.c:4523)
 inet6_ioctl (net/ipv6/af_inet6.c:577)
 sock_do_ioctl (net/socket.c:1190)
 sock_ioctl (net/socket.c:1314)
 __x64_sys_ioctl (fs/ioctl.c:51 fs/ioctl.c:906 fs/ioctl.c:892 fs/ioctl.c:892)
 do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
RIP: 0033:0x7f518fb2de5d
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 73 9f 1b 00 f7 d8 64 89 01 48
RSP: 002b:00007fff14f38d18 EFLAGS: 00000202 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f518fb2de5d
RDX: 00000000200015c0 RSI: 000000000000890b RDI: 0000000000000003
RBP: 00007fff14f38d30 R08: 0000000000000800 R09: 0000000000000800
R10: 0000000000000000 R11: 0000000000000202 R12: 00007fff14f38e48
R13: 0000000000401136 R14: 0000000000403df0 R15: 00007f518fd3c000
 </TASK>

Fixes: fa76c1674f2e ("ipv6: Move some validation from ip6_route_info_create() to rtm_to_fib6_config().")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Reported-by: Yi Lai <yi1.lai@linux.intel.com>
Closes: https://lore.kernel.org/netdev/aBAcKDEFoN%2FLntBF@ly-workstation/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250501005335.53683-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>