vt: keyboard, reorder user buffer handling in vt_do_kdgkb_ioctl

vt: keyboard, reorder user buffer handling in vt_do_kdgkb_ioctl

KDGKBSENT (the getter) needs only 'user_kdgkb->kb_func' from the
userspace, i.e. the index. Then it needs a buffer for a local copy of
'kb_string'.

KDSKBSENT (the setter) needs a copy up to the length of
'user_kdgkb->kb_string'.

That means, we obtain the index before the switch-case and use it in
both paths and:
1) allocate full space in the getter case, and
2) copy the string only in the setter case. We do it by strndup_user
   helper now which was not available when this function was written.

Given we copy the two members of 'struct kbsentry' separately, we no
longer need a local definition. Hence we need to change all the sizeofs
here too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20201029113222.32640-11-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>