netfilter: ipvs: make global sysctl readonly in non-init netns

[ Upstream commit 174c376278949c44aad89c514a6b5db6cee8db59 ]

Because the data pointer of net/ipv4/vs/debug_level is not updated per
netns, it must be marked as read-only in non-init netns.

Fixes: c6d2d445 ("IPVS: netns, final patch enabling network name space.")
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>