mld: fix memory leak in mld_del_delrec()

Similar to the fix done for IPv4 in commit e5b1c6c6
("igmp: fix memory leak in igmpv3_del_delrec()"), we need to
make sure mca_tomb and mca_sources are not blindly overwritten.

Using swap() then a call to ip6_mc_clear_src() will take care
of the missing free.

BUG: memory leak
unreferenced object 0xffff888117d9db00 (size 64):
  comm "syz-executor247", pid 6918, jiffies 4294943989 (age 25.350s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 fe 88 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000005b463030>] kmemleak_alloc_recursive include/linux/kmemleak.h:43 [inline]
    [<000000005b463030>] slab_post_alloc_hook mm/slab.h:522 [inline]
    [<000000005b463030>] slab_alloc mm/slab.c:3319 [inline]
    [<000000005b463030>] kmem_cache_alloc_trace+0x145/0x2c0 mm/slab.c:3548
    [<00000000939cbf94>] kmalloc include/linux/slab.h:552 [inline]
    [<00000000939cbf94>] kzalloc include/linux/slab.h:748 [inline]
    [<00000000939cbf94>] ip6_mc_add1_src net/ipv6/mcast.c:2236 [inline]
    [<00000000939cbf94>] ip6_mc_add_src+0x31f/0x420 net/ipv6/mcast.c:2356
    [<00000000d8972221>] ip6_mc_source+0x4a8/0x600 net/ipv6/mcast.c:449
    [<000000002b203d0d>] do_ipv6_setsockopt.isra.0+0x1b92/0x1dd0 net/ipv6/ipv6_sockglue.c:748
    [<000000001f1e2d54>] ipv6_setsockopt+0x89/0xd0 net/ipv6/ipv6_sockglue.c:944
    [<00000000c8f7bdf9>] udpv6_setsockopt+0x4e/0x90 net/ipv6/udp.c:1558
    [<000000005a9a0c5e>] sock_common_setsockopt+0x38/0x50 net/core/sock.c:3139
    [<00000000910b37b2>] __sys_setsockopt+0x10f/0x220 net/socket.c:2084
    [<00000000e9108023>] __do_sys_setsockopt net/socket.c:2100 [inline]
    [<00000000e9108023>] __se_sys_setsockopt net/socket.c:2097 [inline]
    [<00000000e9108023>] __x64_sys_setsockopt+0x26/0x30 net/socket.c:2097
    [<00000000f4818160>] do_syscall_64+0x76/0x1a0 arch/x86/entry/common.c:296
    [<000000008d367e8f>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 1666d49e ("mld: do not remove mld souce list info when set link down")
Fixes: 9c8bb163 ("igmp, mld: Fix memory leak in igmpv3/mld_del_delrec()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>