mm: gmem: Introduce gmem related madvise opcode

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



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Introduce new madvise opcode in preparation for hmadvise:

  MADV_PREFETCH: prefetch pages for hNUMA node
  MADV_PINNED: pin pages

In order to avoid conflict to existing or new madvise opcode, make the
new one begin with 0x1000.

Signed-off-by: Yang Yanchao <yangyanchao@huawei.com>