9p/xen: do not memcpy header into req->rc

while 'h' is packed and can be assumed to match the request payload,
req->rc is a struct p9_fcall which is not packed and that memcpy
could be wrong.

Fix this by copying each fields individually instead.

Reported-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Suggested-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lkml.kernel.org/r/alpine.DEB.2.22.394.2211211454540.1049131@ubuntu-linux-20-04-desktop
Link: https://lkml.kernel.org/r/20221122001025.119121-1-asmadeus@codewreck.org


Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>