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>