Driver core: fix show_uevent from taking up way too much stack

Declaring an array of PAGE_SIZE does bad things for people running with
4k stacks...

Thanks to Tilman Schmidt for tracking this down.

Cc: Tilman Schmidt <tilman@imap.cc>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>