vsock: fix child netns mode initialization

vsock: fix child netns mode initialization

When a new network namespace is created, vsock_net_init() correctly
initializes the namespace's mode by reading the parent's `child_ns_mode`
via vsock_net_child_mode(). However, the `child_ns_mode` of the new
namespace was always hardcoded to VSOCK_NET_MODE_GLOBAL, regardless of
its own mode.

This means that if a parent namespace has `child_ns_mode` set to "local",
the child namespace correctly gets mode "local", but its `child_ns_mode`
is reset to "global". As a result, further nested namespaces will
incorrectly get mode "global" instead of inheriting "local", breaking
the expected propagation of the mode through nested namespaces.

Fix this by initializing `child_ns_mode` to the namespace's own mode,
so the setting propagates correctly through all levels of nesting.

Fixes: eafb64f40ca4 ("vsock: add netns to vsock core")
Cc: bobbyeshleman@meta.com
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260212205916.97533-2-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>