mm/sharepool: Simplify sp_make_share_k2u()

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



--------------------------------

The 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>