rust: helpers: add is_vmalloc_addr wrapper for NOMMU builds

Commit 47ac2a4b5cd8 ("rust: kvec: implement shrink_to for KVVec")
introduced a call to bindings::is_vmalloc_addr(). However, this
fails to compile on architectures where CONFIG_MMU is disabled,
resulting in the following build error:

    error[E0425]: cannot find function `is_vmalloc_addr` in crate `bindings`
       --> rust/kernel/alloc/kvec.rs:781:32
        |
    781 |         if !unsafe { bindings::is_vmalloc_addr(self.ptr.as_ptr().cast()) } {
        |                                ^^^^^^^^^^^^^^^ not found in `bindings`

When CONFIG_MMU is not set, is_vmalloc_addr() is defined as a
static inline function in <linux/mm.h> that unconditionally
returns false. Because bindgen skips static inline functions
when generating bindings, the symbol is completely missing from
the Rust bindings crate.

Fix this by providing a C helper wrapper, rust_helper_is_vmalloc_addr(),
in rust/helpers/vmalloc.c. This ensures the function is reliably
exposed to Rust regardless of the MMU configuration. On NOMMU builds,
this allows KVVec::shrink_to() to successfully compile and correctly
route all allocations through the kmalloc realloc path.

Fixes: 47ac2a4b5cd8 ("rust: kvec: implement shrink_to for KVVec")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202605220811.LRplxeBR-lkp@intel.com/
Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260523-is-vmalloc-addr-build-fix-v1-1-73c919440c41@zohomail.in
[ Pasted exact compiler output and expanded it. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/rust/helpers/vmalloc.c b/rust/helpers/vmalloc.c
index 326b030487a2..6aed13292313 100644
--- a/rust/helpers/vmalloc.c
+++ b/rust/helpers/vmalloc.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#include <linux/mm.h>
 #include <linux/vmalloc.h>
 
 __rust_helper void *__must_check __realloc_size(2)
@@ -8,3 +9,8 @@ rust_helper_vrealloc_node_align(const void *p, size_t size, unsigned long align,
 {
 	return vrealloc_node_align(p, size, align, flags, node);
 }
+
+__rust_helper bool rust_helper_is_vmalloc_addr(const void *x)
+{
+	return is_vmalloc_addr(x);
+}