netfilter: bpf: must hold reference on net namespace

In the Linux kernel, the following vulnerability has been resolved:

netfilter: bpf: must hold reference on net namespace

BUG: KASAN: slab-use-after-free in __nf_unregister_net_hook+0x640/0x6b0

Read of size 8 at addr ffff8880106fe400 by task repro/72=

bpf_nf_link_release+0xda/0x1e0

bpf_link_free+0x139/0x2d0

bpf_link_release+0x68/0x80

__fput+0x414/0xb60

Eric says:

It seems that bpf was able to defer the __nf_unregister_net_hook()

after exit()/close() time.

Perhaps a netns reference is missing, because the netns has been

dismantled/freed already.

bpf_nf_link_attach() does :

link->net = net;

But I do not see a reference being taken on net.

Add such a reference and release it after hook unreg.

Note that I was unable to get syzbot reproducer to work, so I

do not know if this resolves this splat.