shmem: don't unmap a memrory region in restore_shmem_content()

This function doesn't allocate memory, so it should not release it.

CID 73380 (#1 of 1): Use after free (USE_AFTER_FREE)
14. deref_arg: Calling munmap dereferences freed pointer addr.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>