sw64: fix memmap_range_valid()

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I645QZ



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Fix memmap_range_valid() so users can reserve memory successfully
even part of the requested space extends beyond end of memory.

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