mm: gmem: add gm_dev_destroy

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



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gm_dev_destroy function is needed by some driver's code

Fixes: 848492f2 ("mm: gmem: Introduce vm_object for gmem")
Signed-off-by: Cunshu <Ni&lt;nicunshu@huawei.com>