mm: gmem: Introduce gmem_dev

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



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Introduce gmem_dev to pass ioctl command between userspace & kernel.

Co-developed-by: Ni Cunshu <nicunshu@huawei.com>
Co-developed-by: Feilong Lin <linfeilong@huawei.com>
Co-developed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: luochunsheng <luochunsheng@huawei.com>