Merge branch 'vsock-add-write-once-semantics-to-child_ns_mode'

Merge branch 'vsock-add-write-once-semantics-to-child_ns_mode'

Bobby Eshleman says:

====================
vsock: add write-once semantics to child_ns_mode

Two administrator processes may race when setting child_ns_mode: one
sets it to "local" and creates a namespace, but another changes it to
"global" in between. The first process ends up with a namespace in the
wrong mode. Make child_ns_mode write-once so that a namespace manager
can set it once, check the value, and be guaranteed it won't change
before creating its namespaces. Writing a different value after the
first write returns -EBUSY.

One patch for the implementation, one for docs, and one for tests.

v2: https://lore.kernel.org/r/20260218-vsock-ns-write-once-v2-0-19e4c50d509a@meta.com
v1: https://lore.kernel.org/r/20260217-vsock-ns-write-once-v1-1-a1fb30f289a9@meta.com
====================

Link: https://patch.msgid.link/20260223-vsock-ns-write-once-v3-0-c0cde6959923@meta.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>