rust: io: gate ioremap/iounmap on CONFIG_HAS_IOMEM

s390 does not provide ioremap()/iounmap() when CONFIG_HAS_IOMEM is not
set (which requires CONFIG_PCI on that architecture). This causes a
build failure with Rust enabled on e.g. s390 allnoconfig:

	In file included from rust/helpers/helpers.c:68:
	rust/helpers/io.c:8:9: error: call to undeclared function 'ioremap'; ISO C99 and later do not support implicit function declarations
	      [-Wimplicit-function-declaration]
	    8 |         return ioremap(offset, size);
	      |                ^
	rust/helpers/io.c:19:2: error: call to undeclared function 'iounmap'; ISO C99 and later do not support implicit function declarations
	      [-Wimplicit-function-declaration]
	   19 |         iounmap(addr);

Guard the C helpers behind #ifdef CONFIG_HAS_IOMEM and cfg-gate the Rust
io::mem module, such that IoMem, ExclusiveIoMem and IoRequest are not
available without CONFIG_HAS_IOMEM.

Note that the C API is inconsistent about this. For instance,
devm_ioremap() has no stub and produces a link failure without
CONFIG_HAS_IOMEM, whereas devm_platform_ioremap_resource() provides an
inline stub returning -EINVAL.

The approach taken here (compile-time gating) matches the former, which
is slightly more appropriate since any driver performing MMIO currently
requires CONFIG_HAS_IOMEM.

Ideally, s390 should provide ioremap()/iounmap() stubs unconditionally
(as UML already does), removing the need for any config gating as
discussed in [1]; a follow-up patch for s390 is expected.

Cc: Arnd Bergmann <arnd@arndb.de>
Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://lore.kernel.org/all/20260803180931.97202-1-ojeda@kernel.org [1]
Fixes: 3f70ebe638 ("s390: Enable Rust support")
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260805212920.1996937-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/helpers/io.c

@@ -3,6 +3,7 @@
 #include <linux/io.h>
 #include <linux/ioport.h>
 
+#ifdef CONFIG_HAS_IOMEM
 __rust_helper void __iomem *rust_helper_ioremap(phys_addr_t offset, size_t size)
 {
 	return ioremap(offset, size);
@@ -18,6 +19,7 @@ __rust_helper void rust_helper_iounmap(void __iomem *addr)
 {
 	iounmap(addr);
 }
+#endif /* CONFIG_HAS_IOMEM */
 
 __rust_helper u8 rust_helper_readb(const void __iomem *addr)
 {

rust/kernel/io.rs

@@ -9,6 +9,7 @@
     prelude::*, //
 };
 
+#[cfg(CONFIG_HAS_IOMEM)]
 pub mod mem;
 pub mod poll;
 pub mod register;

rust/kernel/platform.rs

@@ -17,10 +17,7 @@
         from_result,
         to_result, //
     },
-    io::{
-        mem::IoRequest,
-        Resource, //
-    },
+    io::Resource,
     irq::{
         self,
         IrqRequest, //
@@ -31,6 +28,9 @@
     ThisModule, //
 };
 
+#[cfg(CONFIG_HAS_IOMEM)]
+use crate::io::mem::IoRequest;
+
 use core::{
     marker::PhantomData,
     mem::offset_of,
@@ -307,6 +307,7 @@ pub fn resource_by_name(&self, name: &CStr) -> Option<&Resource> {
     }
 }
 
+#[cfg(CONFIG_HAS_IOMEM)]
 impl Device<Bound> {
     /// Returns an `IoRequest` for the resource at `index`, if any.
     pub fn io_request_by_index(&self, index: u32) -> Option<IoRequest<'_>> {