mount: close mnt.ns_fd only for sub-namespaces (v3)

nsid->mnt.ns_fd is initialized into 0, so currently
fini_restore_mntns() closes the 0 descriptor if processes
lives in a current mount namespace (NS_CRIU).

Without this patch I get the following error:
(00.166444)   4109: Inherit fd tty:[8800:d] -> 0 has been closed

v2: typo fix
v3: do nothing when processes are restored in the local mntns
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>