loader: net_open() should not replace f->f_devdata

net_open() does replace f_devdata with pointer to netdev_sock,
this will cause memory leak when device is closed, but also does
alter the devopen() logic.

We should store &netdev_sock to dev->d_opendata instead, this
would preserve and follow the devopen() logic.

Fixes network boot on aarch64 (tested by bz).

Reviewed-by: imp
MFC After: 2 weeks
Differential Revision: https://reviews.freebsd.org/D32227