If ip6_convert_metrics() fails to allocate memory, it should not
overwrite rt->fib6_metrics or we risk a crash later as syzbot found.

BUG: KASAN: null-ptr-deref in atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
BUG: KASAN: null-ptr-deref in refcount_sub_and_test+0x92/0x330 lib/refcount.c:179
Read of size 4 at addr 0000000000000044 by task syzkaller832429/4487

CPU: 1 PID: 4487 Comm: syzkaller832429 Not tainted 4.16.0+ #6
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1b9/0x294 lib/dump_stack.c:113
 kasan_report_error mm/kasan/report.c:352 [inline]
 kasan_report.cold.7+0x6d/0x2fe mm/kasan/report.c:412
 check_memory_region_inline mm/kasan/kasan.c:260 [inline]
 check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
 kasan_check_read+0x11/0x20 mm/kasan/kasan.c:272
 atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
 refcount_sub_and_test+0x92/0x330 lib/refcount.c:179
 refcount_dec_and_test+0x1a/0x20 lib/refcount.c:212
 fib6_info_destroy+0x2d0/0x3c0 net/ipv6/ip6_fib.c:206
 fib6_info_release include/net/ip6_fib.h:304 [inline]
 ip6_route_info_create+0x677/0x3240 net/ipv6/route.c:3020
 ip6_route_add+0x23/0xb0 net/ipv6/route.c:3030
 inet6_rtm_newroute+0x142/0x160 net/ipv6/route.c:4406
 rtnetlink_rcv_msg+0x466/0xc10 net/core/rtnetlink.c:4648
 netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2448
 rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:4666
 netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
 netlink_unicast+0x58b/0x740 net/netlink/af_netlink.c:1336
 netlink_sendmsg+0x9f0/0xfa0 net/netlink/af_netlink.c:1901
 sock_sendmsg_nosec net/socket.c:629 [inline]
 sock_sendmsg+0xd5/0x120 net/socket.c:639
 ___sys_sendmsg+0x805/0x940 net/socket.c:2117
 __sys_sendmsg+0x115/0x270 net/socket.c:2155
 SYSC_sendmsg net/socket.c:2164 [inline]
 SyS_sendmsg+0x29/0x30 net/socket.c:2162
 do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Fixes: d4ead6b3 ("net/ipv6: move metrics from dst to rt6_info")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Ahern <dsa@cumulusnetworks.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>