x86: Change crash kernel to reserve via reserve_early()

use find_e820_area()/reserve_early() instead.

-v2: address Eric's request, to restore original semantics.
     will fail, if the provided address can not be used.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
LKML-Reference: <4B09E2F9.7040403@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>