Preparation. Extract the "search the criu's mount info" code from
resolve_external_mounts() into the new simple helper, find_ext_ns_id().

Also change resolve_external_mounts() to check ext_ns == NULL rather
than !opts.autodetect_ext_mounts. Cosmetic.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>