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

[tsoome]

Summary

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).

[bz]

Is it limited to aarch64 or general EFI netbooting?

[bz]

I think manu replied as well saying he saw this.

[tsoome]

In D32227#727121, @bz wrote:

Is it limited to aarch64 or general EFI netbooting?

We get unexpected data (assuming pointer to devdesc structure), so it really depends on memory state and howthe system will react.

[bz]

Can the other reviewers please comment and this be comitted if dared okay? I just forgot to patch a tree for another new system and ran into it again.