shmem: dump shared memory before dumping namespaces

page_ids is a global variable and it is used to dump shared memroy,
process memory and ipc shared memory.

The problem is that an IPC namespace is dumped in a child process,
so page_ids isn't changed in a parent process. This patch is a quick
workaround this issue.

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>