KEYS: Fix mistaken sizeof call in pgp_key_generate_id

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5H61R


CVE: NA

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pgp_key_generate_id() is trying get the size of a flexible length
structure, however the sizeof() is called on the pointer itself.
Besides, considering it's trying to get the size of a flexible length
structure, use struct_size() instead.

Fixes: 4006f47d ("KEYS: PGP data parser")
Signed-off-by: GUO Zihua <guozihua@huawei.com>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>