mm: gmem: Introduce vm_object in preparation for gmem

euleros inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7WLVX



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Defines a centrailized logical mapping table that reflects the mapping
information regardless of the underlying arch-specific MMUs.

Co-developed-by: Yang Kunlin <yangkunlin7@huawei.com>
Signed-off-by: Liu Chao <liuchao173@huawei.com>