net: Real fix for check_params users

OK, last try: 8e4416af45 broke -net socket, ffad4116b9 tried to fix it
but broke error reporting of invalid parameters. So this patch widely
reverts ffad4116b9 again and intead fixes those callers of check_params
that originally suffered from overwritten buffers by using separate
ones.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
