keys: Fix memory leak in copy_net_ns

If copy_net_ns() failed after net_alloc(), net->key_domain is leaked.
Fix this, by freeing key_domain in error path.

syzbot report:
BUG: memory leak
unreferenced object 0xffff8881175007e0 (size 32):
  comm "syz-executor902", pid 7069, jiffies 4294944350 (age 28.400s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000a83ed741>] kmemleak_alloc_recursive include/linux/kmemleak.h:43 [inline]
    [<00000000a83ed741>] slab_post_alloc_hook mm/slab.h:439 [inline]
    [<00000000a83ed741>] slab_alloc mm/slab.c:3326 [inline]
    [<00000000a83ed741>] kmem_cache_alloc_trace+0x13d/0x280 mm/slab.c:3553
    [<0000000059fc92b9>] kmalloc include/linux/slab.h:547 [inline]
    [<0000000059fc92b9>] kzalloc include/linux/slab.h:742 [inline]
    [<0000000059fc92b9>] net_alloc net/core/net_namespace.c:398 [inline]
    [<0000000059fc92b9>] copy_net_ns+0xb2/0x220 net/core/net_namespace.c:445
    [<00000000a9d74bbc>] create_new_namespaces+0x141/0x2a0 kernel/nsproxy.c:103
    [<000000008047d645>] unshare_nsproxy_namespaces+0x7f/0x100 kernel/nsproxy.c:202
    [<000000005993ea6e>] ksys_unshare+0x236/0x490 kernel/fork.c:2674
    [<0000000019417e75>] __do_sys_unshare kernel/fork.c:2742 [inline]
    [<0000000019417e75>] __se_sys_unshare kernel/fork.c:2740 [inline]
    [<0000000019417e75>] __x64_sys_unshare+0x16/0x20 kernel/fork.c:2740
    [<00000000f4c5f2c8>] do_syscall_64+0x76/0x1a0 arch/x86/entry/common.c:296
    [<0000000038550184>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

syzbot also reported other leak in copy_net_ns -> setup_net.
This problem is already fixed by cf47a0b8.

Fixes: 9b242610 ("keys: Network namespace domain tag")
Reported-and-tested-by:  <syzbot+3b3296d032353c33184b@syzkaller.appspotmail.com>
Signed-off-by: Takeshi Misawa <jeliantsurux@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>