im->tomb and/or im->sources might not be NULL, but we
currently overwrite their values blindly.

Using swap() will make sure the following call to kfree_pmc(pmc)
will properly free the psf structures.

Tested with the C repro provided by syzbot, which basically does :

 socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 3
 setsockopt(3, SOL_IP, IP_ADD_MEMBERSHIP, "\340\0\0\2\177\0\0\1\0\0\0\0", 12) = 0
 ioctl(3, SIOCSIFFLAGS, {ifr_name="lo", ifr_flags=0}) = 0
 setsockopt(3, SOL_IP, IP_MSFILTER, "\340\0\0\2\177\0\0\1\1\0\0\0\1\0\0\0\377\377\377\377", 20) = 0
 ioctl(3, SIOCSIFFLAGS, {ifr_name="lo", ifr_flags=IFF_UP}) = 0
 exit_group(0)                    = ?

BUG: memory leak
unreferenced object 0xffff88811450f140 (size 64):
  comm "softirq", pid 0, jiffies 4294942448 (age 32.070s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 ff ff ff ff 00 00 00 00  ................
    00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000c7bad083>] kmemleak_alloc_recursive include/linux/kmemleak.h:43 [inline]
    [<00000000c7bad083>] slab_post_alloc_hook mm/slab.h:439 [inline]
    [<00000000c7bad083>] slab_alloc mm/slab.c:3326 [inline]
    [<00000000c7bad083>] kmem_cache_alloc_trace+0x13d/0x280 mm/slab.c:3553
    [<000000009acc4151>] kmalloc include/linux/slab.h:547 [inline]
    [<000000009acc4151>] kzalloc include/linux/slab.h:742 [inline]
    [<000000009acc4151>] ip_mc_add1_src net/ipv4/igmp.c:1976 [inline]
    [<000000009acc4151>] ip_mc_add_src+0x36b/0x400 net/ipv4/igmp.c:2100
    [<000000004ac14566>] ip_mc_msfilter+0x22d/0x310 net/ipv4/igmp.c:2484
    [<0000000052d8f995>] do_ip_setsockopt.isra.0+0x1795/0x1930 net/ipv4/ip_sockglue.c:959
    [<000000004ee1e21f>] ip_setsockopt+0x3b/0xb0 net/ipv4/ip_sockglue.c:1248
    [<0000000066cdfe74>] udp_setsockopt+0x4e/0x90 net/ipv4/udp.c:2618
    [<000000009383a786>] sock_common_setsockopt+0x38/0x50 net/core/sock.c:3126
    [<00000000d8ac0c94>] __sys_setsockopt+0x98/0x120 net/socket.c:2072
    [<000000001b1e9666>] __do_sys_setsockopt net/socket.c:2083 [inline]
    [<000000001b1e9666>] __se_sys_setsockopt net/socket.c:2080 [inline]
    [<000000001b1e9666>] __x64_sys_setsockopt+0x26/0x30 net/socket.c:2080
    [<00000000420d395e>] do_syscall_64+0x76/0x1a0 arch/x86/entry/common.c:301
    [<000000007fd83a4b>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 24803f38 ("igmp: do not remove igmp souce list info when set link down")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Hangbin Liu <liuhangbin@gmail.com>
Reported-by: syzbot+6ca1abd0db68b5173a4f@syzkaller.appspotmail.com
Signed-off-by: David S. Miller <davem@davemloft.net>