x86: Fix discontigmem + non-HIGHMEM compile

It's not necessarily a very sane configuration, but people running "make
randconfig" noticed it wouldn't compile.  This fixes some obvious
problems in discontig.c to allow a clean compile.

Acked-by: andrew hendry <andrew.hendry@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>