mm/sharepool: Avoid NULL pointer dereference in mg_sp_group_add_task

ascend inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5DS9S


CVE: NA

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create_spg_node() may fail with NULL pointer returened, and in
the out_drop_spg_node path, the NULL pointer will be dereferenced
in free_spg_node().

Signed-off-by: Yuan Can <yuancan@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>