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>

mount.c

@@ -2276,11 +2276,11 @@ static int do_bind_mount(struct mount_info *mi)
 	char mnt_path_tmp[] = "/tmp/cr-tmpfs.XXXXXX";
 	char mnt_path_root[] = "/cr-tmpfs.XXXXXX";
 	char *root, *cut_root, rpath[PATH_MAX];
-	bool force_private_remount = false;
 	unsigned long mflags;
 	int exit_code = -1;
 	bool shared = false;
 	bool master = false;
+	bool private = false;
 	char *mnt_path = NULL;
 	struct stat st;
 	bool umount_mnt_path = false;
@@ -2299,12 +2299,13 @@ static int do_bind_mount(struct mount_info *mi)
 		 * to proper location in the namespace we restore.
 		 */
 		root = mi->external->val;
-		force_private_remount = mi->internal_sharing;
+		private = !mi->master_id && (mi->internal_sharing || !mi->shared_id);
 		goto do_bind;
 	}
 
 	shared = mi->shared_id && mi->shared_id == mi->bind->shared_id;
 	master = mi->master_id && mi->master_id == mi->bind->master_id;
+	private = !mi->master_id && !shared;
 	cut_root = cut_root_for_bind(mi->root, mi->bind->root);
 
 	if (list_empty(&mi->bind->children))
@@ -2378,9 +2379,9 @@ out:
 	 * shared - the mount is in the same shared group with mi->bind
 	 * mi->shared_id && !shared - create a new shared group
 	 */
-	if (restore_shared_options(mi, force_private_remount || (!shared && !master),
-					mi->shared_id && !shared,
-					mi->master_id && !master))
+	if (restore_shared_options(mi, private,
+	                           mi->shared_id && !shared,
+	                           mi->master_id && !master))
 		return -1;
 
 	mi->mounted = true;