mm: sharepool: fix potential AA deadlock

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5R0X9


CVE: NA

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

Fix a AA deadlock caused by nested lock in mg_sp_group_add_task().

Deadlock path:

mg_sp_group_add_task()

    down_write(sp_group_sem)
    find_or_alloc_sp_group()
	!spg_valid()
	sp_group_drop()
	    free_sp_group() -> down_write(sp_group_sem)
    ---> AA deadlock

Signed-off-by: Guo Mengqi <guomengqi3@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>