9p/fd: Use P9_HDRSZ for header size

stable inclusion
from stable-v5.10.159
commit 1331bcfcac181eaa4eaae82055c79791c02103e3
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7NTXH

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1331bcfcac181eaa4eaae82055c79791c02103e3

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

[ Upstream commit 6854fadb ]

Cleanup hardcoded header sizes to use P9_HDRSZ instead of '7'

Link: https://lkml.kernel.org/r/20221117091159.31533-4-guozihua@huawei.com


Signed-off-by: GUO Zihua <guozihua@huawei.com>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
[Dominique: commit message adjusted to make sense after offset size
adjustment got removed]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>