sw64: fix crash kernel memory reservation

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I73OCN



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Memory for crash kernel is reserved through memblock, so it should be
done before paging init.

Check base and size of requested memory region and make sure it can be
reserved before doing the actual reservation.

Signed-off-by: Mao Minkai <maominkai@wxiat.com>
Reviewed-by: He Sheng <hesheng@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>