mm: gmem: Add gm_as in struct mm_struct

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



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Add gm_as in struct mm_struct in preparation for find corresponding gmem
device used for hmadvise.

Signed-off-by: Weixi Zhu <weixi.zhu@huawei.com>