mount: separate private remount conditions for external and non-external

We have two separate cases 1) mount is external - has mi->external set,
2) mount is internal - mi->external == 0. For those two cases we need
separate decesions on making moutn private.

In both cases we need private remount if:
a)mount does not have master_id and shared_id
or
b)if mount has different shared_id from bind source.

But do not private remount if has master_id or it will be lost. As after
setting private the mount is alone in its shared group so setting master
will silently fail as kernel can not find any for our mount.
(see do_make_slave)

https://jira.sw.ru/browse/PSBM-42829



v2: rework patches 2/4 and 4/4 together and no need then in 3/4
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>