rust: finish using custom FFI integer types

In the last kernel cycle we migrated most of the `core::ffi` cases in
commit d072acda48 ("rust: use custom FFI integer types"):

    Currently FFI integer types are defined in libcore. This commit
    creates the `ffi` crate and asks bindgen to use that crate for FFI
    integer types instead of `core::ffi`.

    This commit is preparatory and no type changes are made in this
    commit yet.

Finish now the few remaining/new cases so that we perform the actual
remapping in the next commit as planned.

Acked-by: Jocelyn Falempe <jfalempe@redhat.com> # drm
Link: https://lore.kernel.org/rust-for-linux/CANiq72m_rg42SvZK=bF2f0yEoBLVA33UBhiAsv8THhVu=G2dPA@mail.gmail.com/
Link: https://lore.kernel.org/all/cc9253fa-9d5f-460b-9841-94948fb6580c@redhat.com/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/seq_file.rs

@@ -36,7 +36,7 @@ impl SeqFile {
             bindings::seq_printf(
                 self.inner.get(),
                 c_str!("%pA").as_char_ptr(),
-                &args as *const _ as *const core::ffi::c_void,
+                &args as *const _ as *const crate::ffi::c_void,
             );
         }
     }