pid namespaces: move alloc_pid() lower in copy_process()

pid namespaces: move alloc_pid() lower in copy_process()

When we create new namespace we will need to allocate the struct pid, that
will have one extra struct upid in array, comparing to the parent.

Thus we need to know the new namespace (if any) in alloc_pid() to init this
struct upid properly, so move the alloc_pid() call lower in copy_process().

Signed-off-by: Pavel Emelyanov <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Sukadev Bhattiprolu <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>