mm/sharepool: Use vm_private_data to store the spa

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EORS


CVE: NA

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When we destroy a vma, we first find the spa depending on the
vma->vm_start, during which we should hold the sp_area_lock.
While we store the spa in vma, we can get the spa directly.
Don't worry if the spa exists or if it's to be freed soon, since
we have increaced the refcount for the spa when it's mappend into
a vma.

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>