rust: redefine bindings::compat_ptr_ioctl in Rust

There is currently an inconsistency between C and Rust, which is that
when Rust requires cfg(CONFIG_COMPAT) on compat_ioctl when using the
compat_ptr_ioctl symbol because '#define compat_ptr_ioctl NULL' does not
get translated to anything by bindgen.

But it's not *just* a matter of translating the '#define' into Rust when
CONFIG_COMPAT=n. This is because when CONFIG_COMPAT=y, the type of
compat_ptr_ioctl is a non-nullable function pointer, and to seamlessly
use it regardless of the config, we need a nullable function pointer.

I think it's important to do something about this; I've seen the mistake
of accidentally forgetting '#[cfg(CONFIG_COMPAT)]' when compat_ptr_ioctl
is used multiple times now.

This explicitly declares 'bindings::compat_ptr_ioctl' as an Option that
is always defined but might be None. This matches C, but isn't ideal:
it modifies the bindings crate. But I'm not sure if there's a better way
to do it. If we just redefine in kernel/, then people may still use the
one in bindings::, since that is where you would normally find it. I am
open to suggestions.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260105-redefine-compat_ptr_ioctl-v1-1-25edb3d91acc@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

rust/kernel/miscdevice.rs

@@ -410,7 +410,7 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
         compat_ioctl: if T::HAS_COMPAT_IOCTL {
             Some(Self::compat_ioctl)
         } else if T::HAS_IOCTL {
-            Some(bindings::compat_ptr_ioctl)
+            bindings::compat_ptr_ioctl
         } else {
             None
         },