network: make Link and NetDev always have the valid poiter to Manager

c4397d94 introduces
link_detach_from_manager() and netdev_detach_from_manager(), and they
set Link::manager or NetDev::manager NULL.
But, at the time e.g. link is removed, hence link_drop() is called,
there may be still some asynchronous netlink call is waiting, and
their callbacks hit assertion.

This make {link,netdev}_detach_from_manager() just drop all references
from manager, but keep the pointer to manager.

Fixes #11411.