mm/sharepool: Simplify sp_unshare_uva()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I650K6



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The unshare process for k2task can be normalized with k2spg
since there exist a local sp group for k2task.

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>