jffs2: fix NULL pointer dereference in jffs2_scan_medium

hulk inclusion
category: bugfix
bugzilla: 185988, https://gitee.com/openeuler/kernel/issues/I4YVV3



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In jffs2_scan_medium,
if `s = kzalloc(sizeof(struct jffs2_summary), GFP_KERNEL);` returns error,
go to "out" to do clear. Null pointer dereference occurs when
`if (s->sum_list_head)` is executed in "out".

Fixes: bf7ba557361f ("[Huawei] jffs2: fix memory leak in jffs2_scan_medium")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>