mount: return a relative path from cut_root_for_bind()

static char *cut_root_for_bind(char *target_root, char *source_root)

Currently it returns a relative path if source_root is '/' and
an absolute path for other cases.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/mount.c

@@ -806,6 +806,8 @@ static char *cut_root_for_bind(char *target_root, char *source_root)
 			break;
 		BUG_ON(target_root[tok] == '\0');
 	}
+	if (target_root[tok] == '/')
+		tok++;
 
 	return target_root + tok;