The entries are only 32-bit, so restrict the virtual address to stay
below 0xffff_ffff.  With KERNELBASE set to 0xc000_0000, this in effect
restricts access to the first 1GB of real memory.

Make setup_kcore conditional on CONFIG_PROC_KCORE for both 32/64.
Signed-off-by: Ed Swarthout <Ed.Swarthout@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>