x86: fix an incompatible pointer type warning on 64-bit compilations

Fix an incompatible pointer type warning on x86_64 compilations.
early_memtest() is passing a u64* to find_e820_area_size() which is expecting
an unsigned long.  Change t_start and t_size to unsigned long as those are
also 64-bit types on x88_64.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>