restore: Block SIGCHLD during root_item initialization

(Was "user_ns: Block SIGCHLD during namespaces generation")

We don't want asynchronous signal handler during creation
of namespaces (for example, in create_user_ns_hierarhy())
as we do wait() synchronous. So we need to block the signal.
Do this once globally.

v2: Set initial ret = 0
v3: Block signal globally in root_item before its children
are created.
v4: Move block to prepare_namespace()

Suggested-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>