mount: cut a common part of sources for bind-mounts

The source of the root mount may be not equal to "/" and we need to take
this fact into account, when we bind-mount it to somewhere.

For example:

11877 ?        Ss     0:00 ./bind-mount --pidfile=bind-mount.pid --outfile=bind-mount.out --dirname=bind-mount.test
11880 ?        Ss     0:00  \_ ./bind-mount --pidfile=bind-mount.pid --outfile=bind-mount.out --dirname=bind-mount.test
[root@avagin-fc19-cr crtools]# cat /proc/11880/mountinfo
68 42 8:3 /root/git/crtools/test / rw,relatime - ext4 /dev/sda3 rw,data=ordered
43 68 0:33 / /proc rw,relatime - proc proc rw
44 68 0:34 / /dev/pts rw,relatime - devpts pts rw,mode=666,ptmxmode=666
45 68 8:3 /root/git/crtools/test/zdtm/live/static/bind-mount.test/test /zdtm/live/static/bind-mount.test/bind rw,relatime - ext4 /dev/sda3 rw,data=ordered

The 45 mount is bind-mount of the 68 mount.
mi(45)->root = /root/git/crtools/test/zdtm/live/static/bind-mount.test/test
mi(68)->root = /root/git/crtools/test

so the comman part is "/root/git/crtools/test" and the command is
mount --bind /zdtm/live/static/bind-mount.test/test /zdtm/live/static/bind-mount.test/bind
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1137,11 +1137,26 @@ static int restore_ext_mount(struct mount_info *mi)
 
 static int do_bind_mount(struct mount_info *mi)
 {
-	char rpath[PATH_MAX];
 	bool shared = mi->shared_id && mi->shared_id == mi->bind->shared_id;
 
 	if (!mi->need_plugin) {
-		snprintf(rpath, sizeof(rpath), "%s%s", mi->bind->mountpoint, mi->root);
+		char rpath[PATH_MAX];
+		int tok = 0;
+
+		/*
+		 * Cut common part of root.
+		 * For non-root binds the source is always "/" (checked)
+		 * so this will result in this slash removal only.
+		 */
+		while (mi->root[tok] == mi->bind->root[tok]) {
+			tok++;
+			if (mi->bind->root[tok] == '\0')
+				break;
+			BUG_ON(mi->root[tok] == '\0');
+		}
+
+		snprintf(rpath, sizeof(rpath), "%s/%s",
+				mi->bind->mountpoint, mi->root + tok);
 		pr_info("\tBind %s to %s\n", rpath, mi->mountpoint);
 
 		if (mount(rpath, mi->mountpoint, NULL,