net: fix UaF in netns ops registration error path

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

net: fix UaF in netns ops registration error path

If net_assign_generic() fails, the current error path in ops_init() tries
to clear the gen pointer slot. Anyway, in such error path, the gen pointer
itself has not been modified yet, and the existing and accessed one is
smaller than the accessed index, causing an out-of-bounds error:

BUG: KASAN: slab-out-of-bounds in ops_init+0x2de/0x320
Write of size 8 at addr ffff888109124978 by task modprobe/1018

CPU: 2 PID: 1018 Comm: modprobe Not tainted 6.2.0-rc2.mptcp_ae5ac65fbed5+ #1641
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.1-2.fc37 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x6a/0x9f
print_address_description.constprop.0+0x86/0x2b5
print_report+0x11b/0x1fb
kasan_report+0x87/0xc0
ops_init+0x2de/0x320
register_pernet_operations+0x2e4/0x750
register_pernet_subsys+0x24/0x40
tcf_register_action+0x9f/0x560
do_one_initcall+0xf9/0x570
do_init_module+0x190/0x650
load_module+0x1fa5/0x23c0
__do_sys_finit_module+0x10d/0x1b0
do_syscall_64+0x58/0x80
entry_SYSCALL_64_after_hwframe+0x72/0xdc
RIP: 0033:0x7f42518f778d
Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48
89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
ff 73 01 c3 48 8b 0d cb 56 2c 00 f7 d8 64 89 01 48
RSP: 002b:00007fff96869688 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
RAX: ffffffffffffffda RBX: 00005568ef7f7c90 RCX: 00007f42518f778d
RDX: 0000000000000000 RSI: 00005568ef41d796 RDI: 0000000000000003
RBP: 00005568ef41d796 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000000
R13: 00005568ef7f7d30 R14: 0000000000040000 R15: 0000000000000000
</TASK>

This change addresses the issue by skipping the gen pointer
de-reference in the mentioned error-path.

Found by code inspection and verified with explicit error injection
on a kasan-enabled kernel.