vsock: Use container_of() to get net namespace in sysctl handlers

vsock: Use container_of() to get net namespace in sysctl handlers

current->nsproxy is should not be accessed directly as syzbot has found
that it could be NULL at times, causing crashes.  Fix up the af_vsock
sysctl handlers to use container_of() to deal with the current net
namespace instead of attempting to rely on current.

This is the same type of change done in commit 7f5611cbc487 ("rds:
sysctl: rds_tcp_{rcv,snd}buf: avoid using current->nsproxy")

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Fixes: eafb64f40ca4 ("vsock: add netns to vsock core")
Link: https://patch.msgid.link/2026022318-rearview-gallery-ae13@gregkh
Signed-off-by: Jakub Kicinski <kuba@kernel.org>