mnt: Remove useless closes in __mntns_get_root_fd()

1)Further mntns_set_root_fd() calls install_service_fd(),
which silently closes already open fd. So, kill close_service_fd()
and make __mntns_get_root_fd() atomical in ROOT_FD_OFF modifications.

2)close_pid_proc() is not need here, as it's about root_item's
/proc directory and __mntns_get_root_fd() actions don't act on it.

v4: New
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/mount.c

@@ -3349,8 +3349,6 @@ int __mntns_get_root_fd(pid_t pid)
 	if (mntns_root_pid == pid) /* The required root is already opened */
 		return get_service_fd(ROOT_FD_OFF);
 
-	close_service_fd(ROOT_FD_OFF);
-
 	if (!(root_ns_mask & CLONE_NEWNS)) {
 		/*
 		 * If criu and tasks we dump live in the same mount
@@ -3389,7 +3387,6 @@ int __mntns_get_root_fd(pid_t pid)
 	}
 
 	fd = openat(pfd, "root", O_RDONLY | O_DIRECTORY, 0);
-	close_pid_proc();
 	if (fd < 0) {
 		pr_perror("Can't open the task root");
 		return -1;