kcm: memory leak in kcm_sendmsg

Hello,

syzkaller has discovered the following memory leak:

unreferenced object 0xffff8800655d5e20 (size 512):
  comm "a.out", pid 10342, jiffies 4295928494 (age 24.051s)
  hex dump (first 32 bytes):
    80 6b 5d 65 00 88 ff ff 69 63 65 73 2f 76 69 72  .k]e....ices/vir
    74 75 61 6c 2f 6e 65 74 2f 74 75 6e 6c 30 2f 71  tual/net/tunl0/q
  backtrace:
    [<0000000017222de2>] kmemleak_alloc_recursive
include/linux/kmemleak.h:55 [inline]
    [<0000000017222de2>] slab_post_alloc_hook mm/slab.h:440 [inline]
    [<0000000017222de2>] slab_alloc_node mm/slub.c:2725 [inline]
    [<0000000017222de2>] __kmalloc_node_track_caller+0x19f/0x360 mm/slub.c:4320
    [<00000000468595b2>] __kmalloc_reserve.isra.39+0x3a/0xe0
net/core/skbuff.c:137
    [<000000005d645735>] __alloc_skb+0x144/0x7c0 net/core/skbuff.c:205
    [<0000000076b4c539>] alloc_skb include/linux/skbuff.h:983 [inline]
    [<0000000076b4c539>] kcm_sendmsg+0x66a/0x2480 net/kcm/kcmsock.c:968
    [<0000000035be3c2b>] sock_sendmsg_nosec net/socket.c:636 [inline]
    [<0000000035be3c2b>] sock_sendmsg+0xd2/0x120 net/socket.c:646
    [<00000000abbae6ad>] SYSC_sendto+0x3de/0x640 net/socket.c:1727
    [<00000000b55ba03b>] SyS_sendto+0x40/0x50 net/socket.c:1695
    [<000000005d14bb62>] entry_SYSCALL_64_fastpath+0x23/0x9a
    [<0000000000cf1810>] 0xffffffffffffffff

unreferenced object 0xffff880053801e40 (size 232):
  comm "a.out", pid 10342, jiffies 4295928494 (age 24.051s)
  hex dump (first 32 bytes):
    c0 20 80 53 00 88 ff ff 00 00 00 00 00 00 00 00  . .S............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000519e860b>] kmemleak_alloc_recursive
include/linux/kmemleak.h:55 [inline]
    [<00000000519e860b>] slab_post_alloc_hook mm/slab.h:440 [inline]
    [<00000000519e860b>] slab_alloc_node mm/slub.c:2725 [inline]
    [<00000000519e860b>] kmem_cache_alloc_node+0x12d/0x2a0 mm/slub.c:2761
    [<000000001a066279>] __alloc_skb+0x103/0x7c0 net/core/skbuff.c:193
    [<0000000076b4c539>] alloc_skb include/linux/skbuff.h:983 [inline]
    [<0000000076b4c539>] kcm_sendmsg+0x66a/0x2480 net/kcm/kcmsock.c:968
    [<0000000035be3c2b>] sock_sendmsg_nosec net/socket.c:636 [inline]
    [<0000000035be3c2b>] sock_sendmsg+0xd2/0x120 net/socket.c:646
    [<00000000abbae6ad>] SYSC_sendto+0x3de/0x640 net/socket.c:1727
    [<00000000b55ba03b>] SyS_sendto+0x40/0x50 net/socket.c:1695
    [<000000005d14bb62>] entry_SYSCALL_64_fastpath+0x23/0x9a
    [<0000000000cf1810>] 0xffffffffffffffff

Reproducer is attached. On 4.15-rc7.