vsock: initialize child_ns_mode_locked in vsock_net_init()

vsock: initialize child_ns_mode_locked in vsock_net_init()

The `child_ns_mode_locked` field lives in `struct net`, which persists
across vsock module reloads. When the module is unloaded and reloaded,
`vsock_net_init()` resets `mode` and `child_ns_mode` back to their
default values, but does not reset `child_ns_mode_locked`.

The stale lock from the previous module load causes subsequent writes
to `child_ns_mode` to silently fail: `vsock_net_set_child_mode()` sees
the old lock, skips updating the actual value, and returns success
when the requested mode matches the stale lock. The sysctl handler
reports no error, but `child_ns_mode` remains unchanged.

Steps to reproduce:
    $ modprobe vsock
    $ echo local > /proc/sys/net/vsock/child_ns_mode
    $ cat /proc/sys/net/vsock/child_ns_mode
    local
    $ modprobe -r vsock
    $ modprobe vsock
    $ echo local > /proc/sys/net/vsock/child_ns_mode
    $ cat /proc/sys/net/vsock/child_ns_mode
    global    <--- expected "local"

Fix this by initializing `child_ns_mode_locked` to 0 (unlocked) in
`vsock_net_init()`, so the write-once mechanism works correctly after
module reload.

Fixes: 102eab95f025 ("vsock: lock down child_ns_mode as write-once")
Reported-by: Jin Liu <jinl@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260401092153.28462-1-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>