mm: remove sharepool sp_unshare_uva current->mm NULL check

ascend inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4ODJ6


CVE: NA

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Remove the unnecessary current->mm NULL check in sp_unshare_uva, and
allow process to unshare kernel mapped addresses in do_exit().

Signed-off-by: Guo Mengqi <guomengqi3@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>