9p/fd: Use P9_HDRSZ for header size

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>