tty/vt: check allocation size in con_set_unimap()

The vmemdup_user() function has no 2-factor argument form. Use array_size()
to check for the overflow.

Signed-off-by: Denis Efremov <efremov@linux.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200603102804.2110817-1-efremov@linux.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>