vt_ioctl: eliminate ret & breaks in vt_ioctl

This is still a leftover from BKL, when we locked it around vt_ioctl's
code. We can return instead of breaks in the switch loop. And we can
return in case of errors too. This allows for sifting of the code to the
left in some cases.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200615074910.19267-29-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>